Heat the olive oil in a pot over medium heat and sweat the onions and garlic. Add the ground beef to the pot, season with salt and pepper, and brown the meat. Stir in the tomato sauce, whole peeled tomatoes, and oregano.
Break the uncooked lasagna noodles into smaller pieces and push them into the sauce. Let the sauce simmer for about 13 minutes until the noodles are al dente.
Stir the ricotta cheese into the sauce.
Place the mozzarella slices on top of the sauce and cover the pot with a lid. Let the one-pot lasagna simmer over low heat until the mozzarella is completely melted and garnish the dish with fresh basil leaves.