Hard-Boiled Egg Gratin In A Bechamel Sauce
Hard-boil the eggs: bring a saucepan of salted water to the boil. Add the eggs, reduce the heat and simmer for 10 minutes. Put the eggs into cold water until just cool enough to handle and then shell. (A trick for shelling hard-boiled eggs is to roll them with the palm of your hand on a hard surface to break up the shell and then peel them under a slow stream of running water.) Reserve the eggs in a bowl of warm water.
Prepare the béchamel sauce: melt the butter in a medium heavy-bottomed saucepan over low heat. Add the flour and whisk for about 2 minutes; do not allow it to colour. Whisk in the milk and bring to the boil, whisking constantly to avoid lumps. Season with the nutmeg and salt and pepper to taste. Reduce the heat and simmer for 10 minutes, whisking constantly and scraping the bottom and sides of the pan to prevent the sauce from sticking. Stir in the cooked onions and cook for 5 minutes.
Heat the grill. Slice the eggs 1 cm (0.4 inch) thick. Spread a thin layer of the béchamel sauce over the bottom of the gratin dish. Layer the egg slices over the béchamel sauce and coat with the remaining sauce. Sprinkle with grated cheese, then grill, turning the dish if necessary to colour evenly, until the top is golden brown. Serve immediately.