Preheat your oven to 375°F (190°C).
Cut the tops off the bell peppers and remove the seeds and membranes. Trim the bottom slightly if needed to make them stand upright, and place them in a baking dish.
Heat the olive oil in a large skillet over medium heat. Add the chopped onion and sauté for about 3–4 minutes until softened. Add the garlic and cook for another minute.
Add the ground beef, breaking it up as it cooks, until browned. Drain any excess fat. Stir in the diced zucchini and mushrooms, cooking for 5 minutes until tender.
Mix in the cooked rice, diced tomatoes, oregano, basil, salt, and pepper. Cook for 2–3 more minutes until everything is combined and heated through.
Generously spoon the beef mixture into each bell pepper, filling them to the top. Sprinkle shredded cheese over each stuffed pepper.
Cover the dish with aluminum foil and bake for 30 minutes. Remove the foil and continue baking for an additional 10–15 minutes until the peppers are tender and the cheese is melted and bubbly.
Let the stuffed peppers cool for a few minutes before serving. Enjoy!