Melt butter and add onions/mushrooms.
Cook about 5 mins.
In a bowl make the custard by mixing cream and eggs.
Stir in cooled off onions/mushrooms, cheese, salt/pepper, prosciutto.
Pour this into the pan.
Cover loosely with nonstick foil and bake 45 mins at 400 degrees.
Remove foil and bake for an additional 10 minutes.
Let cool 10 mins and cut into slices.
Goes well with a nice salad!