Melt butter in large skillet over medium heat. Add potatoes, mushrooms and yellow onion. Sauté until potatoes are crisp-tender, about 10 minutes. Remove from heat; set aside.
Heat oven to 350° F. Mix egg yolks, half-and-half, cayenne and salt in small bowl. Beat egg whites until stiff but not dry.
Spread potato mixture evenly over bottom of a buttered 13 by 9-inch baking dish. Pour egg yolk mixture over potatoes. Spread green pepper, green onions and 3/4 cup of the cheese over egg yolk mixture. Spoon beaten egg whites evenly over all. Sprinkle with remaining cheese.
Bake until puffed and top is golden, about 20 minutes. Serve immediately garnished with parsley.Servings: 6