Make Sauce: Heat olive oil in a skillet over medium heat. Sauté garlic and onion until soft, about 5 minutes. Add crushed tomatoes, tomato paste, basil, oregano, salt, pepper, and sugar. Simmer 15-20 minutes until thickened.
Prepare Filling: Combine ricotta, spinach, egg, parmesan, nutmeg, salt, and pepper in a bowl. Mix until smooth.
Cook Pasta: Boil cannelloni tubes in salted water until al dente, about 8-10 minutes. Drain, rinse with cold water, and pat dry.
Fill Tubes: Transfer filling to a piping bag or zip-top bag with corner cut off. Pipe filling into each cannelloni tube.
Assemble: Preheat oven to 375°F (190°C). Spread a thin layer of sauce in a 9×13 inch baking dish. Arrange filled cannelloni in a single layer. Pour remaining sauce over top and sprinkle with mozzarella.
Bake: Cover with foil and bake 30 minutes. Remove foil and bake 15 minutes more until cheese is golden and bubbly.