Make a smooth dough with the flour, the egg, a pinch of salt and as much water as necessary. Meanwhile, bring a pan of salted water to the boil, and as soon as it boils, drop in small balls of the dough, using a spoon. Cook the gnocchetti for about 15 minutes, drain and serve with the cheese cut into cubes, the Parmigiano and the garlic and sage browned in the butter.