Heat a large nonstick skillet coated with cooking spray over medium-high heat. Add onion and squash; sauté 4 minutes or just until onion begins to brown. Remove from heat.
Stir in 1 cup cheese, corn, and next 8 ingredients. Place mixture in an 11 x 7-inch baking dish coated with cooking spray. Bake at 425° for 25 minutes or until center is set. Top with remaining 1/2 cup cheese. Bake an additional 5 minutes or until cheese melts. Garnish with cilantro, if desired.