Preheat your oven to 375°F (190°C).
In a large skillet, heat olive oil over medium heat and sauté onions until softened.
Add ground beef to the skillet, cook until browned, and season with garlic powder, salt, and pepper.
Stir in the diced bell peppers and cook for 3 minutes.
Add uncooked rice and beef broth to the skillet, stirring well to combine.
Mix in the marinara sauce, then transfer the mixture to a greased casserole dish.
Cover the dish with aluminum foil and bake for 30 minutes.
Remove foil, sprinkle cheese evenly over the casserole, and bake uncovered for another 10 minutes or until the cheese is melted and bubbly.
Let the casserole rest for 5 minutes before serving.