WHISK first 3 ingredients until well blended. Stir in onions and tomatoes.
HEAT 10-inch nonstick skillet sprayed with cooking spray on medium heat. Add egg mixture; cook 4 min. or until almost set, occasionally lifting edge with spatula and tilting pan to allow uncooked portion to flow underneath. Top with ham and cheese; cook 2 to 3 min. or until egg mixture is set and cheese is melted.
SLIP spatula underneath omelet, tip skillet to loosen and gently fold omelet in half. Cut in half.