In a large nonstick skillet, saute the onions, mushrooms and green pepper in 1 tablespoon butter until tender. Remove and keep warm.
In the same skillet, melt remaining butter over medium-high heat.
Whisk the eggs, salt and pepper. Add egg mixture to skillet (mixture should set immediately at edges).
As eggs set, push cooked edges toward the center, letting uncooked portion flow underneath. When the eggs are set, spoon vegetable mixture on one side and sprinkle with cheeses; fold other side over filling. Cut in half.