Step 1: Prepare the Filling

1. If using fresh spinach, sauté it in a dry skillet over medium heat until wilted. Let it cool slightly, then chop finely. If using frozen spinach, ensure it’s thoroughly thawed and excess water is squeezed out.
2. In a large bowl, combine the spinach, ricotta cheese, Parmesan cheese, egg, garlic powder, nutmeg (if using), salt, and pepper. Mix until smooth and well combined.

Step 2: Make the Tomato Sauce

1. Heat olive oil in a medium saucepan over medium heat. Add the onion and garlic, sautéing until soft and fragrant, about 3-4 minutes.
2. Stir in the crushed tomatoes, dried basil, dried oregano, and sugar (if using). Season with salt and pepper. Let the sauce simmer for 15-20 minutes, stirring occasionally, until slightly thickened.

Step 3: Prepare the Pasta

1. Preheat the oven to 375°F (190°C).
2. If using dried cannelloni tubes, parboil them for 3-4 minutes to soften slightly. Drain and let cool. If using fresh lasagna sheets, cut them into rectangles and blanch briefly in boiling water.

Step 4: Assemble the Cannelloni

1. Spread a thin layer of tomato sauce on the bottom of a 9×13-inch baking dish to prevent sticking.
2. Use a piping bag or spoon to fill each cannelloni tube with the spinach ricotta mixture. Place the filled tubes in the baking dish, seam side down if using lasagna sheets.
3. Cover the filled cannelloni with the remaining tomato sauce, spreading it evenly.

Step 5: Add the Toppings

1. Sprinkle shredded mozzarella cheese and grated Parmesan evenly over the top of the sauce.

Step 6: Bake the Cannelloni

1. Cover the dish with aluminum foil and bake for 20-25 minutes.
2. Remove the foil and bake for an additional 15-20 minutes, or until the cheese is melted and golden.

Step 7: Serve

1. Let the dish rest for 5 minutes before serving. Garnish with fresh basil or parsley if desired. Serve hot with garlic bread or a green salad.