Preheat the broiler and set the rack about 4 to 5 inches from the heat source.
Heat oil in large ovenproof nonstick skillet over medium-high heat. Saute mushrooms until browned, seasoning with salt and pepper. Add onion and saute until tender.
Whisk eggs, egg whites, salt, and pepper in a medium bowl. Pour over vegetables in skillet and mix gently. Reduce heat to medium-low, cover skillet, and cook until eggs are set around edges, about 4 to 5 minutes. Sprinkle with cheese and then place skillet under broiler and cook until cheese is melted and just browned, about 2 minutes. Let frittata stand 1 minute before cutting into 8 wedges. Serve with salad on the side.
Combine all ingredients in a large bowl. Toss to coat leaves with dressing and serve with the frittata.