Peel the potatoes and grate them. Wearing gloves to protect against stinging, clean and wash the nettle leaves. After blanching them for a few minutes, drain and chop them up.
Mix the nettles and thoroughly squeezed potatoes in a bowl with a knob of butter softened at room temperature, the eggs, salt and rye flour to obtain a medium consistency dough. Mix well, cover and leave to rest for half an hour.
Meanwhile, slice the onions and fry them gently in about 50 g of butter.
Divide the dough into small pieces and roll them on the floured pastry board. Cut the rolls into 1 cm lengths and gently press them with your thumb, one by one, against the prongs of a fork or against a grater.
Boil the Calhettas in plenty of salted water. As soon as they float to the surface, lift them out with a skimmer and place them in a buttered oven dish. Sprinkle with fried onions, Toma shavings and breadcrumbs. Bake at 180-200°C for 15-20 minutes.
Calhettas (also known as calhettes, calhiettes or cabiettes) are typical gnocchi of the High Valsusa - some people say they originate in Rochemolles, a hamlet of Bardonecchia - are made with raw potatoes. Nettles, which grow in spring, can be replaced with spinach or chard in other seasons.