1. Coarsely grate the potatoes into a bowl. Wash, drain and dry the potatoes on a clean tea-towel. Grate the onion and stir it gently into the potato with seasoning to taste.
2. Melt half the butter in a frying pan, tip the potatoes into the pan and flatten them firmly into a cake. Cook the potatoes fairly gently until the underside is brown and crisp.
3. Using a plate or pan lid to help you, carefully turn the potato cake, adding and heating the remaining butter before sliding the cake back into the pan.
4. Continue cooking the potato cake until the second side is brown and crisp and the potatoes are cooked through. Serve cut into wedges.
For a filling main course, add crisp-fried bacon to the potato cake and serve with a fresh mixed salad.