Coat a 6-inch skillet with cooking spray. Place over medium-high heat until hot. Add zucchini and onions; cook, stirring constantly, until tender. Add tomato, and cook an additional minute. Transfer vegetables to a small bowl; cover, and keep warm. Set aside.
In a medium bowl, beat egg whites at high speed of an electric mixer until stiff peaks form. Combine egg substitute and next 4 ingredients in large bowl. Fold egg whites into egg substitute mixture.
Wipe skillet dry. Coat skillet with cooking spray; place over medium heat until hot. Spread half of egg mixture in skillet. Cover, reduce heat to low. Cook 5 minutes or until puffy. Turn omelet; cook 3 minutes or until golden. Slide omelet onto a serving plate. Spoon half of vegetables over half of omelet; sprinkle with 2 tablespoons cheese. Fold omelet in half. Repeat procedure with remaining ingredients. Serve immediately.