In a small nonstick skillet, saute mushrooms and onion in oil until tender. Remove from skillet and set aside.
In the same skillet, melt butter over medium-high heat. Whisk the eggs, water, 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 mushroom mixture on one side and sprinkle with cheese; fold other side over filling. Slide omelet onto a plate. Serve with marinara sauce.