Finely grate using a roesti grater.
Peel and mince the onion.
Heat a little butter in a frying pan and brown the onion.
Meanwhile, salt the potatoes and mix. Add the fried onions and mix again.
Melt a little more butter then pour the potatoes.
Form a patty using a wooden spatula. Cook 15 min over medium heat without mixing.
Remove from fire.
Place a plate on your pan and turn the roestis on the plate, then pour into the pan to cook the other side.
Cook again 15 minutes and serve.