Instead of tomato sauce, you can take tomatoes, peel them and push them through.
The exact amount of ricotta and spinach may differ. Because your shells can be a little smaller or a little bigger, you can put a little more or a little less filling. In a large pot boil water, add salt and boil cancelloni.When they are ready, drain the water and cool in cold water (to stop process).
Heat the oil, fry the onion for 3-4 minutes, add the garlic and fry for another couple of minutes.
Add the spinach and continue roasting until the leaves curl (about 5 minutes).
When the mixture is ready, drain the liquid and allow to cool.
Then finely chop.
Transfer the spinach to a bowl. Add ricotta, grated Parmesan, salt, pepper, and nutmeg.
Pour half of the tomato sauce into the baking dish.Preheat the oven to 180 degrees.
Fill the shells with the mixture (you can use a cooking bag, you can use a spoon). In the middle of each shell, press a mini mozzarella (or half of it). Put the shells in the mold.
Pour in the remaining tomato sauce.
Sprinkle with cheese.Cover with foil and put in the oven for 20 minutes.
After 20 minutes, remove the foil and put it in the oven for another 15 minutes (or until Golden brown).