Step 1Arrange a rack in center of oven; preheat to 400°. 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.
Step 2Stir in rice and diced tomatoes; season with oregano, salt, and pepper. Let simmer, stirring occasionally, until liquid has reduced slightly, about 5 minutes.
Step 3Arrange peppers cut side up in a 13″ x 9″ baking dish and drizzle with oil. Spoon beef mixture into each pepper. Cover baking dish with foil.
Step 4Bake peppers until tender, about 35 minutes. Uncover, sprinkle with cheese, and continue to bake until cheese is bubbly, about 10 minutes more.
