Cook spinach, onions and garlic in small nonstick skillet sprayed with cooking spray on medium heat 4 min. or until onions are crisp-tender, stirring frequently. Stir in tomatoes.
Whisk eggs and milk until blended. Add to skillet; tilt skillet to evenly coat spinach mixture. Cook 2 to 3 min. or until eggs are almost set, occasionally lifting edge with spatula and tilting skillet to allow uncooked portion to flow underneath. Top with 3 Tbsp. cheese; cook 1 to 2 min. or until eggs are set but top is still slightly moist.
Slip spatula underneath omelet, tip skillet to loosen and gently fold omelet in half. Remove from heat. Sprinkle with remaining cheese; cut in half.