Preheat the oven to 375°F. Slice the tops off the bell peppers and scoop out the seeds and membranes. Use a small knife to carve simple jack-o'-lantern faces into the sides of each pepper.
In a large skillet over medium heat, cook the ground beef with the onion and garlic until the beef is browned and the onion is soft. Drain any excess fat.
Stir the cooked rice, tomato sauce, salt, black pepper, and 1 cup of the cheddar cheese into the skillet until everything is evenly combined.
Spoon the filling into the carved peppers, packing it gently. Set the peppers upright in a baking dish.
Sprinkle the remaining 1/2 cup of cheddar cheese over the tops of the peppers.
Bake for 25 to 30 minutes, until the peppers are tender and the cheese on top is melted and lightly golden.