Preheat oven to 400°F. In a small saucepan, prepare rice according to package directions. In a large skillet over moderate heat, heat oil.
Cook onion till tender for 5 minutes. Stir in tomato paste and garlic and cook till aromatic for a few minute.
Add ground beef and cook, breaking up meat with a wooden spoon, till no longer pink for 6 minutes. Drain fat.
Return beef mixture to skillet, then stir in cooked rice and diced tomatoes. Season with oregano, pepper and salt. Let simmer till liquid has reduced slightly for 5 minutes.
Put peppers cut side-up at a 9x13" baking dish and drizzle with oil. Spoon beef mixture into each pepper and top with Monterey jack, then cover baking dish with foil.
Bake till peppers are tender for 35 minutes. Uncover and bake till cheese is bubbly for 10 minutes longer. Garnish with parsley before serving.