Preheat oven to 400°. In a small saucepan, prepare rice according to package instructions.
Meanwhile, in a large skillet over medium heat, heat oil. Cook onion, stirring occasionally, until softened, about 7 minutes. Stir in garlic and tomato paste and cook, stirring, until fragrant, about 1 minute more. Add ground beef and cook, breaking up meat with a wooden spoon, until no longer pink, about 6 minutes. Drain excess fat.
Return beef mixture to skillet, then stir in rice and diced tomatoes; season with oregano, salt, and pepper. Let simmer, stirring occasionally, until liquid has reduced slightly, about 5 minutes.
Arrange peppers cut side up in a 13"x9" baking dish and drizzle with oil. Spoon beef mixture into each pepper. Top with cheese, then cover baking dish with foil.
Bake peppers until tender, about 35 minutes. Uncover and continue to bake until cheese is bubbly, about 10 minutes more.
Top with parsley before serving.