Now, think of the sauce: melt a bit more than half of the butter in a thick-bottomed pan, then add slowly the flour and the hot milk, using a wooden spoon to stir, and making sure the mix is smooth. Add nutmeg, salt and pepper to taste and let the sauce simmer until it gets thick and velvety. Once it is ready, take it off the stove and let it cool slightly before adding the grated gruyère cheese, the fresh cream and the egg yolk. Mix it all well, then place it aside, making sure to cover it with some cling film first.
Our attention now is all for the scallops: bring a liter of water with a splash of white wine to the boil in a large saucepan. Add salt, pepper, a branch of parsley and the carrot and onion, chopped. Drop the scallops in and let them simmer for 2 minutes. Then separate the white, meaty body of it from the rest, slice it and set it apart. Wash the shells well, dry them and place them on a large oven dish, then put the sliced scallops in them, and cover them with the Mornay sauce. In a mixer, ground the bread with the garlic and some more parsley, then sprinkle the mixture on the scallops. Before baking them, pour over them the remaining butter, melted. Bake at 200 celsius (about 390 fahreneit) for about 10 minutes.