Preheat oven to 350 degrees F. Lightly spray an 8x8 baking dish with cooking spray.
Toast the bread until crisp, then pulse or crush into fine breadcrumbs.
Spread the broccoli evenly in the bottom of the baking dish, then layer the chicken on top of the broccoli.
In a bowl, whisk together the Greek yogurt, light mayonnaise, lemon juice, garlic powder, onion powder, salt, and pepper.
Stir in half of the shredded cheese.
Pour the sauce evenly over the chicken and broccoli.
Sprinkle the remaining cheese on top and add the breadcrumbs.
Bake for 30 minutes, or until bubbly and lightly golden.