Heat the broiler. Position the rack 4 inches from the heat source.
Lightly coat a heavy, ovenproof skillet with cooking spray. Place on the stove over medium heat and add peppers and mushrooms. Saute until vegetables are tender, about 5 minutes.
In a small bowl, whisk together the egg whites and 1/2 cup of the salsa. Pour the egg white mixture into the skillet with the vegetables and cook until partially set, about 5 minutes. Don't attempt to blend or scramble the mixture. Sprinkle with cheddar cheese.
Place the skillet under the broiler and cook until the cheese is melted and eggs are set, about 5 minutes.
Divide the frittata in half and spoon onto individual plates. Garnish each serving with 1 tablespoon salsa and serve immediately.