Saute onion and mushrooms in nonstick, ovenproof 12" skillet over medium-high heat until soft and most of liquid from mushrooms has cooked off, about 10 minutes. Add spinach leaves and saute an additional 5 minutes. Remove from heat.
Whisk eggs together with cheeses and remaining ingredients.
Pour egg mixture into skillet of spinach and mushroom mixture, stirring to combine.
Bake for 15 minutes or until set. Let stand 5 minutes before cutting into 8 wedges.