Sweet Potato Rösti

• 3-4 sweet potatoes, weighing about 250-300 g each
• 1 onion, peeled and thinly sliced

Put the sweet potatoes in a saucepan, cover with water and bring to the boil, then simmer for 15 minutes, so they are partly cooked. Drain and leave to cool - under the cold tap is quickest. Peel the potatoes by scraping off the skin with a knife, then grate or shred them with a mandolin, a grater or the grating attachment of a food processor. Put into a bowl and mix with the sliced onion and s