From Kaiti’s Laughing Raven Kitchen:
Halibut Mornay
Prepare the fish:
Allowing 1/2 lb. of halibut per person, cut fish into 2 in. cubes. Shake cubes with flour seasoned with salt and pepper. Quickly brown the floured fish in olive oil in a hot fry pan. The object is to brown, but not cook the fish. Arrange cubes in a single layer in a 9X9 or 9X12 casserole pan.
Prepare a Mornay Sauce:
This makes about 1 cup of Sauce, so double it for the larger pan of halibut.
Melt 4 T. unsalted butter over low heat. Chop and sauté a small yellow onion and 2 garlic cloves over low heat. Then slowly blend in 2 T. flour. Slowly add 1 c. whole milk, mixing well so there are no lumps. Turn off the heat after the sauce has thickened slightly and strain it through a colander. Add a little of the sauce to 1 egg yolk mixed with 2 T. cream, then return this mixture to the rest of the sauce and blend it well there should be no lumps. Add a grating of nutmeg and splash of white wine if desired. Grate and stir in about 4 heaping Tablespoons of cheese. We often use a mixture of Parmesan and Gruyere, but cheddar is good too. (Feel free to experiment with one of your favorite cheeses and let us know which one you like best.)
Assemble and bake the dish:
Pour the sauce over the fish, trying to cover all the fish pieces. This helps keep the fish moist when it is baked. Grate some more cheese on top of the sauce, then cover and bake for about 40 minutes at 350 degrees. If you have a meat thermometer, the fish should be about 135 degrees internally when you take it out of the oven. Serve while still steaming hot.