Seafood au Gratin: Poach the Seafood, Then Broil It for Six Minutes

Seafood au gratin is where the rubbery-casserole problem gets expensive. You are combining three or four things that each cook in under three minutes — shrimp, scallops, flaky white fish, maybe crab — and committing all of them to the same half hour in the oven. The scallops lose by the widest margin, going from sweet and barely set to small white erasers at twenty dollars a pound.

The version I make now barely goes in the oven at all. The seafood poaches in milk, gently, for two minutes each, pulled out in stages as it finishes. That milk then becomes the sauce, so nothing is wasted and everything the seafood gave up ends up back on the plate. Assembly happens in a warm pan, and the dish spends six minutes under the broiler — long enough to blister the cheese and nothing else.

It is not a slow-baked dish pretending to be fast. It is a fast dish that looks slow-baked at the table.

Ingredients

A shallow 2-quart gratin dish or a 10-inch skillet. Shallow matters twice: more surface to brown, and nothing deep enough to need oven time.

For the Poach and the Sauce

• 3 cups whole milk
• 1 shallot, halved
• 1 bay leaf
• 3 sprigs thyme
• 6 black peppercorns
• 5 tablespoons unsalted butter
• 1/4 cup all-purpose flour
• 4 ounces Gruyère, grated
• 1/3 cup grated Parmesan
• 2 teaspoons Dijon mustard
• 2 tablespoons dry sherry
• 1 1/4 teaspoons kosher salt
• Pinch of cayenne
• 1 tablespoon fresh lemon juice

For the Seafood

• 3/4 pound large shrimp, peeled and deveined
• 3/4 pound sea scallops, side muscle removed, halved crosswise
• 1/2 pound cod or haddock, cut into 1 1/2-inch chunks
• 8 ounces lump crab meat, picked over and drained

For the Crust

• 3/4 cup panko
• 2 ounces Gruyère, grated
• 2 tablespoons unsalted butter, melted
• 2 tablespoons chopped chives

How to Make It

Prep: 35 minutes  |  Bake: 8 minutes  |  Serves: 6

1. Put the milk, shallot, bay leaf, thyme, and peppercorns in a wide saucepan and bring it up until it steams with a few bubbles at the edge. Never let it boil. Hold it there 10 minutes, then fish out the aromatics.
2. Poach in stages, lifting each batch onto a paper-towel-lined tray: the cod first, 2 minutes, until it just begins to flake; then the shrimp, 2 minutes, until pink and curled into a loose C; then the scallops, 90 seconds only, until the outside turns from glassy to white.
3. Pour the poaching milk through a fine strainer into a measuring cup. You should have about 2 1/2 cups; top it up with fresh milk if you are short.
4. Wipe the pan, melt the 5 tablespoons of butter over medium heat, whisk in the flour, and cook 90 seconds. Add the strained milk in three additions, whisking each smooth, and simmer 4 minutes until it coats a spoon.
5. Off the heat, stir in both cheeses, the Dijon, sherry, salt, cayenne, and lemon juice. Heat the broiler with a rack 6 inches below the element. Toss the panko, Gruyère, and melted butter together for the crust.
6. Fold the poached cod, shrimp, and scallops into the warm sauce, then the crab last and most gently — it was never poached and does not need to be, so it only has to be coated. Pour everything into the gratin dish.
7. Scatter the crust over and broil 5 to 6 minutes, turning the dish once, until the surface is blistered and deep brown in patches. Watch it the whole time — the gap between golden and burnt is about forty seconds.
8. Rest 5 minutes, scatter the chives, and serve. The sauce tightens noticeably in those five minutes, so do not panic if it looks loose coming out.

Can You Make It the Night Before?

Reheat the sauce gently with a splash of milk before the seafood goes in — a cold béchamel is stiff enough to tear the fish apart when you fold. It should be loose and warm, about the texture of heavy cream.

If you genuinely need it done in advance, the compromise I would accept is poaching the seafood, cooling it uncovered on a tray, and refrigerating it separately from the sauce for up to a day. Fold them together cold, broil 8 minutes instead of 6, and accept firmer scallops. Do not freeze it in any configuration — poached scallops in dairy come out grainy and the sauce splits.

The Broiler Is Doing a Different Job Than the Oven

A 375°F oven heats a casserole all the way through, which is what you want for raw potatoes and exactly what you do not want for seafood that is already cooked. A broiler is directional. It puts intense heat on the top inch and leaves the rest of the dish mostly alone.

That is why the sauce goes in warm and the seafood goes in cooked. There is nothing left for the middle of the dish to accomplish, so only the surface needs to change — and six minutes of top heat gives you the blistered, patchy, dark-gold crust that is the whole visual appeal of a gratin.

Turn the dish once halfway through. Almost every home broiler has a hot spot, and the result otherwise is one burnt corner and one pale one.

Why the Crab Goes In Last and Uncooked

Lump crab is sold cooked and pasteurized, so poaching it would be a third cooking and would break the lumps apart in the pan besides. It goes into the finished sauce purely to be coated and warmed.

Fold it in at the very end with three or four strokes and then stop, even if it looks unevenly distributed. It evens out when you spread the mixture in the dish. Stirring to distribute crab is how crab stops being crab.

This is also why the crab goes in after the poached fish rather than with it. By then the sauce has cooled slightly from the cold seafood folded through it, so the crab meets warm sauce instead of hot sauce, and warm is all it needs.

Scaling It Up Without Losing the Method

The constraint when you double this is not the oven, it is the broiler — and specifically the amount of surface that sits close enough to the element to brown. Doubling into one deep dish defeats the whole recipe: a four-inch-deep gratin cannot be heated through by top heat alone, so you would have to bake it, and baking it is what we are avoiding. Two shallow dishes, broiled one after the other, is the only version of doubling that works.

Poach in batches too, and keep them small. Dropping two pounds of cold shrimp into the milk at once will take it from bare steam down to lukewarm, and then it climbs back up while the shrimp sit in it — which is a slow poach, the opposite of what you want. Half a pound at a time keeps the milk steady, and the whole exercise is faster than it sounds because nothing stays in for more than two minutes.

The sequencing that works for eight or ten people: poach everything, make one batch of sauce for both dishes, fill both, then broil the first while the second waits on the counter. Cover the finished one loosely with foil and it will still be properly hot six minutes later when the second comes out. Do not try to broil both at once on two racks — the lower dish gets no color and you end up leaving it in long enough to overcook the scallops.

Questions People Ask

Why are my scallops rubbery in a gratin?

Because they finished cooking before the dish went in the oven and kept going. A sea scallop is done in about ninety seconds of gentle poaching; after that the proteins contract and squeeze out water. In a baked gratin they spend half an hour past done.

Can I use frozen seafood?

Yes, and most scallops and shrimp in the case were frozen anyway. Thaw overnight in the refrigerator, then dry everything well — surface water dilutes the poaching milk, and the sauce is made from that milk. Avoid anything labeled wet-packed.

What if I don’t have a broiler?

Bake it at 475°F on the top rack for 10 to 12 minutes instead. You get less blistering and a slightly drier result, so add a quarter cup of cream to the sauce for margin. A toaster oven’s broil setting works well for a dish this shallow.

Can I add potatoes or pasta to stretch it?

Pasta yes, potatoes carefully. Fold in a pound of cooked small shells with the seafood and increase the sauce by half. For potatoes, cook them fully separately — par-cooked potatoes need real oven time, which is what this recipe avoids.

Do leftovers reheat at all?

Not really, and I would plan around that. Everything in the dish is cooked to the edge of done, so any reheating takes it past. What I do with leftovers is eat them cold the next day, or warm a portion very gently in a small pan on the stove with a splash of milk, stopping while it is barely hot. The microwave is the one thing that reliably turns the scallops to rubber.