Preheat the oven to 375 degrees. In a bowl, whisk the egg whites until frothy.
In another large bowl, whisk the eggs until foamy and lemon colored. Fold the egg whites into the eggs. Add the mushrooms and season with salt and pepper.
Heat a nonstick ovenproof skillet slicked with olive oil or nonstick cooking spray over medium heat. Add the eggs, sprinkle the sage leaves on top, and cook for about 4 minutes, or until the edges are set.
Transfer to the oven and cook for 4 to 6 minutes, until set and golden. Cut into wedges and serve over the mixed greens.