Preheat and Prepare Bell Peppers: Preheat your oven to 350°F (175°C). Slice the bell peppers in half vertically and remove the stems, seeds, and membranes. Place them cut side up in a microwave-safe 13×9-inch baking dish. Add 2 tablespoons of water to the bottom of the dish and cover lightly with parchment paper. Microwave for about 3 minutes, just until the peppers begin to soften slightly. Drain off any excess water.
Brown the Beef and Sauté the Aromatics: In a large skillet or Dutch oven, heat 1 tablespoon of olive oil over medium-high heat. Add the ground beef and diced onion, cooking and stirring until the beef is browned and fully cooked, about 7–8 minutes. Drain off any rendered fat to avoid a greasy filling.
Create the Savory Filling: Reduce the heat to medium and stir in the cooked rice, tomato sauce, Worcestershire sauce, Italian seasoning, sugar, garlic powder, and onion powder. Season with salt and pepper to your taste. Mix in ½ cup of cheddar and ½ cup of Monterey Jack or mozzarella, stirring until the cheese is melted and everything is well combined. Taste and adjust seasoning if needed.
Fill and Bake the Peppers: Lightly season the inside of each pepper half with salt and pepper. Spoon the beef and rice mixture evenly into the pepper halves. Drizzle with the remaining 1 tablespoon olive oil. Cover the baking dish with foil and bake in the preheated oven for 35 minutes, until the peppers are tender and the filling is hot throughout.
Add Cheese Topping and Finish Baking: Remove the foil, sprinkle the stuffed peppers with the remaining ½ cup of cheddar and ½ cup of Monterey Jack or mozzarella. Return to the oven and bake uncovered for an additional 10 minutes, or until the cheese is bubbly and melted.
Serve and Garnish: Remove from the oven and let rest briefly before serving. Top with chopped parsley or basil for a fresh, flavorful finish.