Coat a 10-inch nonstick skillet with cooking spray; place over medium-high heat until hot. Add mushrooms and onions; sauté until tender. Remove from pan; set aside.
Add spinach to pan; sauté until spinach wilts. Remove from heat; stir in cream cheese. Remove from pan, and keep warm. Wipe pan with a paper towel. Combine egg substitute and next 3 ingredients, stirring well.
Coat pan with cooking spray; place over medium heat until hot. Pour egg mixture into pan. As mixture begins to cook, lift edges with a spatula, and tilt pan to allow uncooked portion to flow underneath. When set, spoon mushroom mixture, spinach mixture, and Cheddar cheese over half of omelet. Loosen with spatula, and fold in half. Cook 1 to 2 minutes or until cheese begins to melt. Slide onto a plate; cut in half. Serve immediately.