Preheat oven to 350 degrees F. Spray a 9x13-inch baking dish with nonstick cooking spray; set aside.
Melt butter in a large skillet over medium-high heat, add onion and sauté for a few minutes until tender. Mix in flour and season with salt, pepper, and onion powder then cook for an additional 1-2 minutes.
Gradually whisk in chicken broth and milk and cook for 3 minutes, or until thickened. Remove from heat and mix in sour cream and 3⁄4 cup bread crumbs until well combined.
In a large mixing bowl, mix together cream sauce, chicken and zucchini until evenly coated and spread in prepared pan. Sprinkle remaining 1⁄4 cup bread crumbs on top and spray generously with nonstick cooking spray.
Bake for 50 minutes then turn oven to high broil and cook an additional 1-2 minutes or until golden brown on top.