Preheat the oven to 375°F (190°C). Lightly grease a baking dish and set aside.
Prepare the peppers by cutting off the tops and removing seeds and membranes. Place peppers upright in the baking dish.
Heat olive oil in a large skillet over medium heat.
Add ground beef and cook until browned, breaking it apart as it cooks.
Season with salt, garlic powder, onion powder, and black pepper.
Stir in tomato sauce and let simmer for 2–3 minutes.
Add cooked rice and mix until well combined.
Stir in 1 cup mozzarella cheese, then remove from heat.
Fill each bell pepper generously with the beef mixture.
Top with remaining mozzarella cheese.
Cover loosely with foil and bake for 15 minutes.
Remove foil and broil for 2 minutes, or until cheese is melted and lightly golden.
Remove from oven and let rest for 5 minutes before serving.