Begin by preheating the oven to 400°F. In a small saucepan, cook the rice according to the instructions on the package.

While the rice is cooking, heat oil in a large skillet over medium heat. Sauté the onion until softened, stirring occasionally, for approximately 7 minutes.

Add garlic and tomato paste, and continue to cook, stirring, until the mixture becomes fragrant, about 1 minute. Add the ground beef and cook until it is no longer pink, breaking it up with a wooden spoon as it cooks, for about 6 minutes. Drain any excess fat from the skillet.

Next, incorporate the cooked rice and diced tomatoes into the skillet. Season the mixture with oregano, salt, and pepper. Allow it to simmer, stirring occasionally, until the liquid has slightly reduced, approximately 5 minutes.

Arrange the bell peppers in a 13"x9" baking dish with the cut side facing up. Drizzle the peppers with oil. Fill each pepper with the beef mixture. Top each pepper with cheese, and then cover the baking dish with foil.

Place the baking dish in the preheated oven and bake the peppers until they become tender, which should take about 35 minutes. Remove the foil and continue baking until the cheese is bubbly, for an additional 10 minutes.