Add spinach to boiling water and cook for 6 minutes. IF yours is fresh, add spinach to the boiling one and cook for about 7 minutes, you will see that it "boils down".
Boil the pasta in half (if the instructions say 12 minutes, then it means only 6 minutes). Cannellone, large shells and similar paste can be used ... Rinse with cold water and arrange gently on a plate, board or whichever suits you best. Leave in a bowl, stick together.
Use a blender to combine riccotta and spinach, pepper and salt to taste. Place the filling in a pastry bag and start filling the paste.
In a baking dish, brush with béchamel sauce. Put the stuffed pasta in a dish, top with the béchamel sauce, sprinkle with grated cheese on top.
Cook in a preheated oven at 180 degrees for 20 minutes (no need to cover). Done.