1. Saute onions and mushrooms in butter in skillet over medium heat for about 3-5 minutes.
2. In a large bowl, beat together the eggs, milk, and pepper.
3. Stir in cheese and the mushroom mixture and bacon.
4. Pour into a greased 2-quart rectangular baking dish (12x7-1/2x2-inch).
5. Bake in 350 degree F oven for 35-40 minutes or until puffed and a knife inserted in center comes out clean.
6. Let stand 5-10 minutes before cutting.
7. Serve warm.