Au Gratin Potatoes: A Cheese Sauce That Does Not Break and a Crust That Does

The two things that go wrong with au gratin potatoes are a grainy, greasy sauce and a crust that never happens. They look unrelated. They are both about where the heat is.

Cheese added to a pan still sitting over a burner tightens and squeezes out its fat, which is the grit and the slick you get in a broken sauce — and there is no stirring it back together. Meanwhile a dish piled three inches deep in a tall casserole has almost no surface relative to its volume, so it steams rather than browns and you end up with a pale lid over a lot of potato.

Both are easy to avoid. The cheese goes in off the heat. The dish goes in wide and shallow. The word gratin refers to the browned crust, not to the potatoes, so the crust is not a garnish here — it is the thing the dish is named after.

Ingredients

Use a wide shallow dish, around 3 quarts and no more than 2 inches deep. A 9×13 works; a deep casserole does not.

For the Potatoes

• 3 pounds Yukon Gold potatoes, peeled
• Butter, for the dish

For the Cheese Sauce

• 4 tablespoons unsalted butter
• 1 small shallot, minced
• 1/4 cup all-purpose flour
• 3 cups whole milk, warmed
• 1 cup heavy cream
• 1 bay leaf
• 1/4 teaspoon freshly grated nutmeg
• 2 teaspoons kosher salt
• 1/2 teaspoon black pepper
• 8 ounces Gruyère, grated from a block
• 4 ounces aged white cheddar, grated from a block
• 1/2 teaspoon Dijon mustard

For the Gratin Crust

• 1 cup panko breadcrumbs
• 1/2 cup finely grated Parmesan
• 2 tablespoons unsalted butter, melted

How to Make It

Prep: 30 minutes  |  Bake: 70 minutes  |  Serves: 8 to 10

1. Heat the oven to 375°F and butter a wide, shallow 3-quart baking dish.
2. Melt the butter in a large saucepan over medium heat and cook the shallot 3 minutes until soft. Sprinkle in the flour and stir constantly for 90 seconds.
3. Whisk in the warm milk in four additions, then the cream, bay leaf, nutmeg, salt, and pepper. Simmer 5 minutes, until it coats the back of a spoon. Remove the bay leaf.
4. Take the pan off the heat. Add the Gruyère and cheddar in four handfuls, stirring until each disappears before the next goes in, then the Dijon. Taste it and push the salt a shade past comfortable — three pounds of plain potato is about to dilute it.
5. Slice the potatoes 1/8 inch thick, as evenly as you can. Fold them directly into the warm sauce in the pot, which coats every slice and leaves no dry pockets. Layering by hand and pouring sauce over always leaves some.
6. Spread it into the dish no deeper than 2 inches, then press the top layer flat with the back of a spoon so the slices lie down in the sauce instead of curling up out of it. Curled edges burn before the crust browns.
7. Cover tightly with foil and bake 45 minutes, until a knife meets no resistance in the center.
8. Toss the panko with the Parmesan and melted butter. Uncover the dish, scatter the crumbs evenly over the top, and bake another 25 minutes, until the crust is deep golden brown and the sauce is bubbling up at the edges.
9. Rest 15 minutes before serving — the sauce is loose straight from the oven and sets into something sliceable as it cools.

Can You Make It the Night Before?

A cheese sauce that has been properly emulsified reheats well — the flour in the béchamel is what holds it together, and it is far more forgiving than a plain cream sauce. What does not survive is the crust. Panko that has been refrigerated under foil goes soft and never fully re-crisps, so if you care about the top, add it fresh on reheat day.

You can make the sauce on its own up to two days ahead. It will set into something close to a solid in the refrigerator; warm it gently in a saucepan with a splash of milk, stirring, until it pours again. Do not boil it, and do not microwave it on full power, which separates it in under a minute.

Freezing works for about two months, with one caveat: Gruyère and aged cheddar both tolerate the freezer, but the potato slices come back slightly grainy at the edges. Thaw overnight in the refrigerator rather than on the counter, and reheat covered so the surface does not dry out before the middle warms.

Au Gratin vs. Scalloped: What the Words Actually Mean

A gratin is defined by its top. The word comes from the French for the browned, crusty layer you scrape off a dish, and anything baked under enough top heat to form that crust can be gratinéed — cauliflower, pasta, onion soup. Potatoes au gratin, in American usage, means potato slices in a cheese sauce, finished with a crust of cheese, breadcrumbs, or both. There is an actual cheese sauce in the dish, and there is a deliberate crust on it.

Scalloped potatoes have neither. They are potatoes baked in plain cream or milk, thickened by nothing but the starch the potato slices release, with no cheese inside and nothing crunchy on top. That is not a lesser dish — it is a quieter one, and it is seasoned very differently because there is no aged cheese doing the heavy lifting. Restaurant menus use the two names interchangeably, which is how most of us learned them wrong.

There is a wrinkle worth knowing, which is that the French original runs the other way. Gratin dauphinois, the dish from the Dauphiné that all of this descends from, is classically potatoes, milk or cream, garlic and nutmeg — and traditionally no cheese at all, with the crust coming from the cream and starch browning on top. By that standard the American “scalloped potato” is the closer relative, and what we call au gratin is nearer to a gratin savoyard, which does bring cheese. None of that makes the American usage wrong, but it does explain why the two names have been tangled together for a century.

Yukon Gold Beats Russet Here, and That Is Backwards

In a scalloped dish you want starchy russets, because the potato’s own starch is the only thickener in the pan. In an au gratin you have a flour-and-cheese sauce doing that job already, which frees you to choose a potato for texture instead — and that changes the answer. Yukon Golds sit between waxy and starchy. They hold their slices distinct instead of blurring into the sauce, they taste faintly buttery, and they will not shed enough starch to turn an already-thick béchamel into paste.

Russets still work, and plenty of good recipes use them, but the result is softer and the layers lose definition. Genuinely waxy potatoes — red bliss, new potatoes — are the one real mistake: they stay firm and slick, and the sauce slides off them rather than clinging. On thickness, 1/8 inch is right again, for the same reason as always. Uneven slices cook unevenly, and in a 2-inch gratin the thick ones at the bottom are the ones still crunching when the crust is ready.

One thing both dishes share is that the slices should go into the sauce rather than the sauce going over the slices. Hand-layering looks tidier and photographs better, but it reliably leaves dry pockets in the middle of the dish where a slice is touching another slice instead of sauce. Those pockets stay firm and chalky while everything around them goes tender. Folding the raw slices into the pot coats all of them at once, and you can still arrange the top layer neatly afterward for the sake of appearances.

Questions People Ask

What is the difference between au gratin and scalloped potatoes?

Au gratin has a cheese sauce and a browned crust on top. Scalloped potatoes are baked in plain cream or milk with no cheese and no topping, thickened only by the starch from the potatoes themselves. The word gratin refers to the crust, which is why a dish without one is not really a gratin at all.

Why is my cheese sauce grainy or oily?

The cheese went in over direct heat, or it came pre-shredded from a bag. Heat makes cheese proteins tighten and expel fat, which is the grit and the slick. Bagged shreds are dusted with starch or cellulose so they do not clump, and that coating never dissolves smoothly. Off the heat, from a block, a handful at a time.

Can I use only cheddar?

Yes, if it is aged — a sharp or extra-sharp block. Mild cheddar disappears into this much milk and cream. Aged cheddar is also slightly more prone to splitting than Gruyère, so be especially careful to add it off the heat.

Do I have to use breadcrumbs?

No, but you do need something that browns. A cheese-only gratin works well: hold back 3 ounces of the Gruyère and scatter it on for the final 25 minutes. What does not work is leaving the top bare, which gives you a dish of potatoes in cheese sauce — good, but not a gratin.

Should I cover it for the whole bake?

No — covered for the first 45 minutes, uncovered for the last 25. The foil traps steam, which is what cooks the potato slices through evenly without the top drying out. Taking it off is what lets the crumbs brown and the sauce reduce at the edges. Leave the foil on the whole time and you get tender potatoes under soft, pale crumbs.