Heat a large skillet over medium heat. Add the ground beef, diced onion, and minced garlic. Cook for 5-6 minutes, breaking the beef apart as it cooks, until the beef is browned and the onion is softened. Drain any excess grease.
Transfer the beef mixture to a 9x13 casserole dish.
Add the diced bell peppers, white rice, diced tomatoes, beef broth, Italian seasoning, salt, pepper, and ¾ cup mozzarella cheese to the casserole dish.
Stir everything together until evenly combined.
Cover tightly with foil and bake for 40 minutes, or until the rice is tender and most of the liquid has been absorbed.
Remove the foil and sprinkle the remaining ¾ cup mozzarella cheese over the top.
Bake uncovered for another 5-10 minutes, until the cheese is melted.
Let the casserole rest for 5 minutes before serving.
Cover tightly while baking: The rice needs the steam from the covered casserole dish to cook properly and absorb the liquid.
Use uncooked white rice: Add the rice dry to the casserole dish. It will cook in the broth and tomato mixture as the casserole bakes.
Drain the beef if needed: If your ground beef releases a lot of grease, drain it before transferring it to the baking dish.
Let it rest before serving: Resting for a few minutes helps the rice finish absorbing any extra liquid and makes the casserole easier to scoop.
Check the rice before adding the cheese: If the rice is not tender after 40 minutes, cover it again and bake for another 5-10 minutes before adding the remaining cheese.