Preheat oven to 350°F and grease a 9x13-inch baking dish.
In a large skillet over medium-high heat, brown the ground beef with the chopped onion until the meat is fully cooked and the onion is softened, about 8-10 minutes. Drain excess fat.
Add the diced bell peppers to the skillet and cook for 5 minutes until slightly softened.
Stir in the cooked rice, tomato sauce, diced tomatoes, garlic powder, Italian seasoning, salt, and pepper. Mix until well combined.
Pour half of the mixture into the prepared baking dish and spread evenly.
Sprinkle 1 cup of mozzarella cheese over the mixture, then top with the remaining beef and rice mixture.
Cover with foil and bake for 30 minutes.
Remove foil, top with remaining 1 cup of mozzarella cheese, and bake uncovered for 15 minutes more until cheese is melted and bubbly.