Saute onion and mushrooms in hot oil in an ovenproof, nonstick, 10 inch skillet over medium-high heat for 10 min or until tender. Stir in spinach, and saute 3 min or until water evaporates and spinach wilts. Remove from heat, and set aside.
Whisk together 4 large eggs, 6 egg white, cheddar cheese, and next 5 ingredients.
Pour egg mixture into skillet with onion mixture, stirring to combine.
Bake at 350 for 12-15 min. or just until set. Let stand for 3 min. Cut into wedges. Garnish with pepper and parsley, if desired.