Preheat the oven to 400°. In a small saucepan (or rice cooker), prepare rice according to package instructions.
Chop the onion.
In a large skillet over medium heat, heat the oil. Cook the onion until soft, about 5 minutes. Stir in the tomato paste and garlic and cook until fragrant, about 1 minute more. Add the ground beef and cook, breaking up the meat with a wooden spoon, until it is no longer pink, about 6 minutes.
Stir in the cooked rice and diced tomatoes. Season with oregano, salt, and pepper. Let simmer until the liquid has reduced slightly, about 5 minutes.
Cut off the tops of the peppers. Cut them in half and use a spoon or ice cream scoop to scoop out the seeds and cores.
Place peppers cut side-up in a 9x13 baking dish and drizzle with oil. Spoon the beef mixture into each pepper and top with cheese, then cover the baking dish with foil.
Bake until peppers are tender, about 35 minutes. Uncover and bake until the cheese is bubbly, 10 minutes more.
Top with sour cream or avocado and enjoy!