For the mushroom casserole, roast the onion until translucent. Add mushrooms and roast for 10 minutes until liquid has evaporated. Remove from heat.
Bring milk to a boil, mix flour with 4 tablespoons cold water, add to boiling milk and cook for 3 minutes. Pour milk into bowl, gradually stir in yolks.
Mix in cheese, season, add mushrooms and stiffly beaten snow. Pour the mixture into a greased pan and bake at 200 degrees for 20 minutes.
Turn down to 180 degrees, cover with aluminum foil and bake for another 20 minutes. Sprinkle the mushroom casserole with chives and roasted mushrooms.