Coat a small nonstick skillet with cooking spray; place over medium-high heat until hot. Add onion; saute 2 minutes. Add zucchini and yellow squash; saute 3 minutes or until squash and onion are tender. Remove from heat; stir in parsley and next 3 ingredients.
Combine egg substitute and milk in a small bowl, stirring with a wire whisk until smooth. Add milk mixture and 3 tablespoons cheese to squash mixture; stir well. Spoon mixture into a 2-cup casserole coated with cooking spray. Sprinkle squash mixture with remaining 1 tablespoon cheese. Bake, uncovered, at 350° for 40 minutes or until set.