Preheat the oven to 350°F. Cut off the tops of the bell peppers and set the pepper "lids" aside for later. Remove the core and seeds from the insides and place the peppers upright in a casserole dish.
Heat some olive oil in a pan over medium heat and cook the ground beef. Stir in the onions, garlic, and tomato sauce, season with salt and pepper, and let simmer until the meat has browned.
Mix the ricotta cheese, egg, basil, and parmesan together.
Cut each lasagna noodle in half, for a total of 16 pieces. Start layering the peppers, starting with a spoonful of the ground beef filling.
Top the ground beef layer with a lasagna noodle, followed by a spoonful of the cheese filling, followed by another lasagna noodle. Continue layering until you reach the top of the pepper, with the final layer being the ground beef filling. Sprinkle mozzarella cheese on top.
Put the "lids" back on the peppers, put the dish in the preheated oven, and cook for 30-35 minutes.