Prepare the ingredients:
Cut the fish fillet into pieces. Wash seafood if it’s frozen or vacuum-packed.
Fry the leeks:
In a saucepan, melt the butter and add the leeks and garlic. Stir, avoiding browning.
Degrease with white wine:
Add the white wine and reduce the amount by half.
Prepare the sauce:
Add the flour or cornstarch, mix, then add the cream. Let it thicken slightly. Season with salt and pepper, and add a squeeze of lemon.
Cook the fish:
Add the fish pieces, shrimp, and mussels to the sauce. Simmer for 5 to 8 minutes, enough time to cook the fish.
Arrange in casserole dishes:
Spread the mixture into 4 small ovenproof dishes. Sprinkle with grated cheese, if desired.
Gratin (optional):
Grill for 5 minutes until golden.
Serve warm, sprinkled with fresh parsley, with rice or steamed vegetables.
