Combine eggs, spinach and tomato in medium bowl. Spray large nonstick skillet with cooking spray; heat over medium-high heat until hot. Add egg mixture; cook 2 to 4 minutes or until eggs are almost set, stirring occasionally. Sprinkle with salt and pepper, if desired.
Divide egg mixture into 4 separate portions in pan. Evenly top with cheese and beef. Remove from heat and cover. Let stand 1 to 2 minutes or until cheese is melted.
Place one egg portion on bread bottoms. Close sandwiches; serve immediately.