Sauté the shallots with a knob of butter and add the well-wrung boiled spinach, cook and remove from heat. Let them cool and add to the sieved ricotta. Mix all the ingredients well, in the meantime cook the paccheri for 10 minutes, drain them, fill them with a sac à poche and place them on a baking dish covered with béchamel sauce. Cover with béchamel and grated mozzarella. Bake at 180 degrees for 15 minutes. Remove from the oven and serve with a generous sprinkling of chopped walnuts.