Preheat oven to 375°F (190°C).
Cut the tops off the bell peppers and remove seeds. Place them in a baking dish, ready for filling.
Heat a large skillet over medium-high heat. Add ground beef and cook until browned, about 7 minutes. Drain excess fat.
Add onion and garlic to the skillet, cooking until softened, about 3 minutes. Stir in cooked lentils, tomato sauce, oregano, basil, salt, and black pepper. Let simmer for 5 minutes.
Stuff each bell pepper with the beef and lentil mixture. Top with shredded mozzarella cheese.
Bake in the preheated oven for 25 minutes, or until peppers are tender and cheese is bubbly and golden.