Preheat oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish.
In a large mixing bowl, combine Greek yogurt, milk, garlic, onion powder, salt, and pepper to make the sauce.
Add chicken, steamed broccoli, cooked quinoa or rice, and shredded cheese to the sauce. Mix until well combined.
Transfer the mixture into the prepared baking dish. Sprinkle breadcrumbs on top if using.
Bake for 25-30 minutes, or until the casserole is bubbly and golden on top.
Let cool slightly before serving.