Preheat your oven to 375°F (190°C). Lightly grease a baking dish that is deep enough to hold the peppers upright. Wash the bell peppers and pat them dry with a towel.
Using a sharp paring knife, carefully slice off the tops of the peppers (keep them for later). Remove the seeds and membranes from the inside. On one side of each pepper, carefully carve out two triangles for eyes and a jagged mouth. Work slowly to ensure the pepper walls don’t crack.
Heat the olive oil in a large skillet over medium heat. Add the diced onion and cook until softened, about 5 minutes. Add the ground meat and cook until browned, breaking it up with a spoon. Stir in the garlic and cook for another minute until fragrant.
Drain any excess fat from the skillet. Stir in the cooked rice, tomato sauce, oregano, salt, and pepper. Let the mixture simmer for 2-3 minutes so the flavors meld together. Remove from heat and stir in half of the shredded cheese.
Spoon the meat and rice mixture into each carved pepper, pressing down gently to fill them to the top. Stand the peppers upright in the prepared baking dish. Place the pepper “lids” back on top.
Cover the baking dish with foil and bake for 25 minutes. Remove the foil, take off the pepper lids, and sprinkle the remaining cheese over the filling. Replace the lids and bake for another 5-10 minutes until the peppers are tender and the cheese is melted. Let them cool for a few minutes before serving.