Preheat oven to 325 and grease casserole dish with butter. In skillet, saute mushrooms & onion in the butter. Add the herbs and cook for about 4 minutes. In a bowl, combine the cream, egg and the egg yolks. Sprinkle the swiss cheese all over the bottom of the casserole, put mushroom mixture on top, then pour cream/egg mixture over that. Bake 30 minutes or until browned & bubbly.