Make the ragù following the instructions.
Wash the potatoes and boil them with their skins in lightly salted water for about 20 minutes, taking care not to overcook them. Drain and when cool enough to handle, peel them.
Pass the potatoes through a food mill, into a wide bowl. Add a little salt and just enough flour to make a soft and elastic dough, kneading with your hands. The dough should be soft and slightly sticky, but it shouldn’t stick onto your work surface.
Meanwhile, bring a large saucepan of salted water to the boil. Dust your hands and work surface lightly with flour. Working quickly, break off pieces of the dough, and roll out into long cylindrical strands of about 1 cm (½ in) in diameter. Cut these strands into pieces at about 2 cm (¾ in) intervals with a sharp knife. Put onto a lightly floured tray until you are ready to cook.
Cook the gnocchi in batches in the boiling water. When they float to the surface they are ready. Lift them out with a slotted spoon or a wire strainer, allowing the water to drain off well. Keep the gnocchi in a hot, buttered oven dish until you are ready to serve them. Transfer to pasta bowls, and add a few tablespoons of hot ragù and a nut of butter. Sprinkle with Parmesan cheese and serve immediately.