Preheat your oven to 375°F (190°C) and lightly grease a 9x9 inch baking dish.
Arrange the cored bell peppers upright in the prepared baking dish.
Heat olive oil in a large skillet over medium-high heat. Add the diced onion and sauté for 3 minutes until soft.
Add the ground beef and minced garlic. Cook, breaking up the meat, until browned and cooked through. Drain any excess fat.
Stir in half of the tomato sauce, salt, and black pepper. Let the mixture simmer for 2 to 3 minutes.
Spoon the ground beef mixture evenly into the hollowed-out bell peppers.
Pour the remaining tomato sauce over the tops of the stuffed peppers.
Cover the baking dish tightly with aluminum foil and bake for 35 minutes.
Remove the foil, sprinkle the shredded cheese evenly over each pepper, and return to the oven uncovered for 5 minutes until the cheese is melted and bubbly.
Garnish with chopped fresh parsley before serving.