1. Melt 3 Tbsp. butter in a 10-inch ovenproof skillet over medium-high heat; add chopped zucchini, squash, and onion, and sauté 12 to 14 minutes or until onion is tender. Remove skillet from heat.
2. Whisk together eggs and next 3 ingredients until well blended. Pour over vegetable mixture in skillet.
3. Bake at 350° for 33 to 35 minutes or until edges are lightly browned and center is set. Sprinkle evenly with chopped fresh basil.