Clean the spinach (if using fresh) and boil in salted water for 2-3 minutes. Drain and wring out in a towel to remove any excess water. Pass through food mill or purée in a food processor.
On a work surface, quickly mix the spinach purée with flour, potato starch, beaten egg, drained fresh ricotta, grated Parmigiano and a pinch of freshly ground nutmeg to form a smooth, even dough.
Set dough aside to rest for 5 minutes on a well floured surface. Using a pastry bag with a smooth ¾ inch tip, form long ropes of dough. Cut into 1-inch pieces.
Melt butter in a large pan over medium heat. Add sage leaves. Remove from heat before butter begins to brown.
Meanwhile, briefly boil dumplings in a large pot of salted water. As soon as they float to the surface, remove with a slotted spoon. Toss with butter, sage and grated cheese until combined. Plate and garnish with tomato sauce (heated with a few basil leaves).
The cheese's aroma and flavor along with the slight sweetness of the egg and flour make the dish tasty and appealing. A white wine that is straw-yellow in color with a bouquet of fresh fruit and hints of aromatic herbs; dry, mellow and hot with freshness supported by its flavor and a good persistence. EST! EST!! EST!!! Di Montefiascone D.O.C.
Spinach are always to be cooked in boiling salt water because in this manner they maintain their green color. The dough must be kneaded quickly to prevent the flour from becoming gummy. The water must boil slowly to prevent the gnudi from falling apart.