Preheat oven to 375 degrees Fahrenheit. Line a baking tray with parchment paper, sprayed with non-stick cooking spray.
Combine ground chicken, egg, garlic, onion powder, Italian seasoning, crushed red pepper flakes, whole wheat breadcrumbs, 1/4 cup of marinara sauce, grated Parmesan cheese, and spinach in a medium-sized mixing bowl.
Using a cookie scoop or bare hands, form into 20 meatballs and place on a baking dish. Bake for 15 minutes.
While meatballs are baking, spray a 9×13 casserole dish with non-stick cooking spray and spread 3/4 cup of marinara sauce to the dish.
Once the meatballs are done cooking, transfer to the baking dish. Top with the marinara sauce left and cheese.
Bake for an additional 15 minutes or until cheese is golden brown. Serve warm.