Dice the onions and ham, sear them with the mushrooms in the pan and let them cool.
Mix the cream with creme fraiche. Layer the schnitzel and the braised ingredients in a baking dish, sprinkle the cheese on top and pour the cream over them. Cover everything with aluminum foil and put in the refrigerator for 24 hours.
Then bake at 250 degrees for 90 minutes, covered with aluminum foil.
You can also bake the casserole straight away without letting it steep for 24 hours.