Peel the onion and grate it finely. Clean and wash the leek and cut it into very thin strips. Wash and peel the potatoes and grate them medium to fine. Immediately after grating, place the potatoes on a cheese cloth and squeeze out the juices into a bowl. As soon as the potato starch begins to settle on the bottom of the bowl, pour out the liquid. Add the grated potatoes to the bowl with the starch.