Melt butter in a small nonstick omelet pan over medium-high heat; add mushrooms and shallot. Cook 5 minutes or until mushrooms are tender; stir in 1/4 teaspoon salt and 1/4 teaspoon pepper. Remove from heat; keep warm.
Combine egg, egg substitute, 1/4 teaspoon salt, and 1/4 teaspoon pepper in a bowl; stir with a whisk..
Wipe pan with paper towels; coat with cooking spray, and place over medium heat. Pour half of egg mixture into pan. As mixture starts to cook, gently lift edges of omelet with a spatula, and tilt pan so uncooked portion flows underneath. Spoon half of mushroom mixture over omelet; top with 1/4 cup cheese. Loosen omelet with spatula; fold in half. Slide omelet onto a plate; keep warm. Repeat procedure with remaining egg mixture, mushroom mixture, and cheese.