In small bowl, combine eggs, milk, onions and salt; beat with fork until well blended. Melt margarine in 9 or 10-inch skillet or omelet pan over medium-high heat; tilt skillet to coat.
Pour egg mixture into skillet; reduce heat to low. Cook 5 to 7 minutes or until egg mixture is set but top is still moist, lifting edges occasionally to allow uncooked egg mixture to flow to bottom of skillet.
Top omelet with chicken, cheese and tomato. With spatula, loosen edge of omelet and fold in half as omelet slides from skillet to serving plate.