Melt butter in a 12-inch ovenproof skillet over medium-high heat; add zucchini and onion, and sauté 12 to 14 minutes or until onion is tender. Remove from heat, and stir in 2 tablespoons grated Parmesan cheese.
Whisk together egg substitute and next 3 ingredients at least 1 minute or until well blended. Pour over vegetable mixture.
Bake at 350° for 13 to 15 minutes or until set; increase oven temperature to broil, and broil 5 1/2 inches from heat 1 to 2 minutes or until edges are lightly browned. Sprinkle evenly with remaining 2 tablespoons grated Parmesan cheese and basil. Garnish, if desired.