Sauté the onion and garlic in olive oil over medium heat until fragrant and translucent, about 3-5 minutes. Add ground beef and cook until browned, stirring occasionally.

Combine the cooked beef mixture with spinach, ricotta cheese, and grated parmesan in a large bowl. Mix until well incorporated.

Fill the cannelloni tubes with the beef and spinach mixture using a spoon or piping bag, packing them tightly.

Spread a layer of tomato pasta sauce in a baking dish, then arrange the filled cannelloni in a single layer on top.

Cover the filled tubes with remaining tomato sauce and sprinkle with shredded mozzarella cheese.

Bake in a preheated oven at 375°F (190°C) for 35 minutes, covered with aluminum foil.

Uncover and bake for an additional 10-15 minutes until cheese is golden and bubbly.