Preheat your oven to 375°F (190°C).

Bring a large pot of salted water to a boil, then add the manicotti shells. Cook until al dente, about 6-8 minutes. Drain and cool on a towel.

In a skillet, heat olive oil, add diced onion and minced garlic. Sauté until onions are translucent, about 3-4 minutes.

Add ground beef to the skillet and cook until browned, about 5-7 minutes. Stir in crushed tomatoes and Italian seasoning, simmer for 10 minutes.

In a mixing bowl, combine ricotta, mozzarella, and Parmesan cheese. Season with salt and pepper to taste.

Stuff each manicotti shell with the cheese mixture using a spoon or piping bag.

Spread a thin layer of meat sauce in a baking dish. Arrange stuffed manicotti on top.

Pour remaining meat sauce over the manicotti and sprinkle with additional mozzarella cheese.

Cover the baking dish with aluminum foil and bake for 40-45 minutes. Remove foil in the last 10 minutes to brown cheese.

Let the manicotti cool for 5-10 minutes before serving.