2 tbsp. extra-virgin olive oil, plus more for drizzling
1 medium onion, chopped
2 tbsp. tomato paste
3 cloves garlic, minced
1 lb. ground beef
1 (14.5-oz.) can diced tomatoes
1 1/2 tsp. dried oregano
6 bell peppers, tops and cores removed
1 cup grated cheese
Freshly chopped parsley, for garnish
Preheat the oven to 400°. In a small saucepan, prepare rice according to package instructions. In a large skillet over medium heat, heat oil. Cook onion until soft, about 5 minutes. Stir in tomato paste and garlic and cook until fragrant, about 1 minute more. Add ground beef and cook, breaking up meat with a wooden spoon, until no longer pink, 6 minutes. Drain fat.
Return beef mixture to skillet, then stir in cooked rice and diced tomatoes. Season with oregano, salt, and pepper. Let simmer until the liquid has reduced slightly, about 5 minutes.
Place peppers cut side-up in a baking dish and drizzle with oil. Spoon beef mixture into each pepper and top with Monterey jack, 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.