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