Cook rice in beef broth according to package directions, about 15-18 minutes, then fluff and set aside.
Preheat oven to 375°F (190°C). Cut tops off peppers, remove seeds and membranes, rinse, and pat dry.
Brown ground beef with onion and garlic in a large skillet, breaking into crumbles, until no pink remains.
Add Italian seasoning, garlic powder, onion powder, smoked paprika, salt, and pepper to the beef and stir to combine.
Stir in drained diced tomatoes and 1 cup (240ml) of tomato sauce, then add cooked rice and mix well.
Remove from heat and stir in ¾ cup (85g) of shredded mozzarella until melted and combined.
Stuff each bell pepper with the filling, packing it firmly, and place upright in a 9x13 inch baking dish.
Spoon remaining tomato sauce over each pepper, then top with the remaining mozzarella and cheddar cheese.
Cover tightly with foil and bake at 375°F (190°C) for 35 minutes, then uncover and bake 10-15 minutes more until cheese is golden and bubbly.
Rest for 5 minutes before serving. Garnish with parsley or red pepper flakes if desired.