Hard eggs, peel and cut in half.
Separate yolks and mash with a fork.
Clean mushrooms and cut into small cubes by hand or possibly in a blitz chopper.
Peel onion, cut into small cubes.
Heat butter in a frying pan.
Sauté the onion, add the mushrooms and cook until the liquid has evaporated.
Finally, season with salt, pepper, chopped parsley. Cool the onion-mushroom mixture a little and then mix with the egg yolks.
Fill this quantity into the halved eggs.
For the béchamel sauce: in a saucepan, cream the butter, brown the flour in it
and gradually pour in hot milk (a quarter to 1/2 L of milk), whisk until smooth and cook to desired consistency.
Season with salt, pepper and freshly grated nutmeg.
Butter a gratin dish and sprinkle with a tiny bit of bread crumbs.
Place the stuffed eggs in the dish and cover with the béchamel sauce.
Bake in the oven heated to 200 °C for about 10-15 minutes.
Serve with a celery salad: peel a celery bulb and cut it into strips.
Boil in water seasoned with a little sugar, salt and the juice of a lemon.
Chop a small onion and mix it with the cooked, still warm celery.
Marinate with vinegar and oil, mix well and cool.
Mix the stuffed and gratinated eggs with the celery salad.