1. Sauté onion and mushrooms in hot oil in an ovenproof nonstick 10-inch skillet over medium-high heat 10 minutes or until tender. Stir in spinach, and sauté 3 minutes or until water evaporates and spinach wilts. Remove from heat, and set aside.
2. Whisk together 4 large eggs, 6 egg whites, Cheddar cheese, and next 5 ingredients.
3. Pour egg mixture into skillet with onion mixture, stirring to combine.
4. Bake at 350° for 12 to 15 minutes or just until set. Let stand 3 minutes. Cut into wedges. Garnish, if desired.
*1 (10-oz.) package chopped frozen spinach, thawed and well drained, may be substituted for fresh spinach.
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