Place a baking sheet in oven. Preheat oven to 475° (leave pan in oven).
Combine first 5 ingredients in a large bowl, stirring with a whisk. Add squash and onion; toss gently to coat. Sprinkle vegetable mixture with 1/4 teaspoon salt and 1/4 teaspoon pepper. Carefully remove preheated pan from oven; coat pan with cooking spray. Arrange vegetable mixture in a single layer on pan.
Bake at 475° for 20 minutes or until tender, turning once. Sprinkle with remaining 1/4 teaspoon salt, remaining 1/4 teaspoon pepper, and chopped parsley.