Preheat oven to 400 degrees. Add salt and pepper to egg whites and whip into soft peaks. In a heavy, oven safe frying pan or cast iron skillet, heat the butter over high heat and sauté mushrooms until soft. Place tomato slices over mushroom. Quickly fold cheese into egg white mixture and pour on top of mushroom/tomato mixture. Place pan in oven and bake for approximately 8 minutes. Remove from oven and flip soufflé over onto serving plate.