Preheat the oven to 350°F (175°C).
Cut a thin slice from the top (stem end) of each bell pepper and remove seeds and membranes. Rinse well. If needed, slice a small portion from the bottom so peppers stand upright.
In a skillet over medium heat, cook the ground beef with chopped onion and garlic until the meat is browned. Drain excess fat if needed.
Stir in the cooked rice, salt, and half of the tomato sauce. Mix until well combined.
Spoon the mixture evenly into each prepared bell pepper.
Place stuffed peppers upright in a baking dish. Pour remaining tomato sauce over the top.
Cover loosely with foil and bake for about 30 minutes.
Remove foil, sprinkle mozzarella cheese over each pepper, and bake uncovered for an additional 10–15 minutes, until cheese is melted and peppers are tender.
Remove from oven and let cool slightly before serving.
Use different colored peppers for variety and sweetness.
Add herbs like parsley or Italian seasoning for extra flavor.
Refrigerator: Store in an airtight container for up to 3 days.
Freezer: Freeze cooked peppers for up to 2 months.