Hollowed-out peppers packed with seasoned ground beef and melted cheese — it's a complete high protein dinner in an edible bowl. Customize the filling however you want.
Preheat oven to 375°F. Carefully slice the tops off the bell peppers and remove seeds and membranes, making sure not to tear the sides.
Brush the outside and inside of each pepper with olive oil. Arrange peppers standing upright in a 9x13-inch baking dish and bake uncovered for 10 minutes to soften.
Meanwhile, heat olive oil in a large skillet over medium heat. Add onion and sauté until translucent, about 3-4 minutes. Add minced garlic and cook for 30 seconds until fragrant.
Add ground beef to the skillet, breaking it up with a spatula. Cook over medium-high heat until browned but still moist, about 5-6 minutes. Stir in dried oregano, salt, black pepper, cooked brown rice, and tomato sauce; mix thoroughly, then simmer for 2-3 minutes to absorb flavors.
Remove skillet from heat and fold in half the shredded mozzarella cheese. Spoon the hot beef mixture evenly into each pepper, packing the filling fully. Top each with the remaining cheese.
Cover the baking dish with foil and bake for 20 minutes. Uncover and bake another 8-10 minutes until cheese is melted and lightly golden. Let rest 5 minutes before serving so peppers hold their shape.