Cook the potaotes and lower them whole in their skins into a pan of salted boiling water, bring back to the boil and simmer for 10-15 minutes until just soft. Test with a sharp knife - you should have to push the knife in, it should not slide in easily, otherwise the potatoes will be overcooked and mushy and will have absorbed too much water. Peel them quickly, as the cooler they get, the less fluffy they become.Hold them in a tea towel to peel as they are hot.
Using a mouli on a medium setting, press the potatoes into a bowl. Pass the potato through the mouli a second time, letting it fall on to the work surface. This second pressing is to make sure that the mixture is lump free, and also lets more air in. If you don't have a mouli, you could use a potato ricer, but only if it has small holes, and you may need to push the potatoes through three times to get the right texture.
Make a hollow in your pile of potatoes, then pour in the egg and sprinkle over some of the flour. Start to blend everything with your hands, adding more flour but as little as you can get away with (you want the flavour of the potato to come through, rather than that of the flour).Work carefully and quickly, as the more you handle the dough, the harder and bouncier it will become. You need the same lightness you would use for pastry.
You should now have a soft dough that holds together, doesn't feel sticky and can be easily shaped. Before you progress, check the dough by cooking a few gnocchi to see how they perform (see Valentina's tip, right).
Divide the dough into 3 equal pieces. Roll a piece at a time into long, thumb-nail thick cylinders on a lightly floured surface, again working lightly and quickly. As you roll you will also be gently stretching the dough. Keep the surface well floured as you don't want the gnocchi to stick.
Cut the dough into thumb-nail long lengths. Some people don't bother to shape and pattern them, but just cook them as they are. However, the shaping and patterning gives a hollow on one side and a pattern on the other that enables the sauce to cling better, and also makes each piece recognisable as a gnocco (a single gnocchi).
Roll the gnocchi in a little flour. Holding them very lightly, form each into a small concave gnocchi shape: hold them against the prongs of the back of a fork, pressing only firmly enough to get the imprint (not so firmly that they go through the prongs), then guide each one so it tumbles away from the fork. Use your thumb as a guide and your fingers to pick and curl the gnocchi up. Spread them on a large board until required.
Bring a large, deep pot of salted water to the boil. Working with a few at a time (don't cook more than you can cope with at once, see tip, right), drop in the gnocchi and listen for the wonderful kissing noise they make as they go in. Let them cook for 2 minutes, during which time they will bob back up to the surface, then scoop them out with a slotted spoon. Taste - they should be sofficí e leggeri (soft and light), the gnocchi equivalent of al dente.
For the tomato sauce, deseed and finely chop the tomatoes. Heat a thin layer of olive oil in a frying pan. Rub about 30 sage leaves in your hand to release the flavour, then fry for a few seconds until they darken slightly. Lift out and drain on paper towel. For each person put 20 gnocchi in a bowl and scatter over the tomatoes and sage. Drizzle over a little melted butter, then finish with a grating of black pepper and a sprinkling of finely grated parmesan.