Clean the spinach, making sure to eliminate all the stems, and rinse well under cold running water. Place in a large saucepan with a knob of butter and the garlic and cook covered with only the water that clings to the leaves.
Add salt and remove garlic. Squeeze and remove remaining water, then chop finely. Blend the spinach, ricotta and grated Parmesan cheese in a bowl. Use a spoon to fill the cannelloni with the mixture.
Butter an oven dish large enough to contain all the cannelloni in a single layer. Spread a ladleful of béchamel sauce on the bottom of the oven dish and arrange the cannelloni in the dish.
Cover with the rest of the béchamel sauce, sprinkle with more Parmesan cheese and bake in the oven. Turn on the grill and cook for a few more minutes to brown.