Simmer the potatoes in their skins in salted water until three-quarters cooked. It is important that they remain slightly firm as they will be grated and cooked a second time. Remove the potatoes from the water and refrigerate before peeling. Heat 20 ml. clarified butter in a pan, add the onion and bacon and sweat without colouring. Remove from the pan and allow to cool. Peel the potatoes and grate on the largest blade of the grater to create long strips of potato.
Preheat the oven to 180°C. Gently mix the onion and bacon through the potato and season with salt and freshly ground pepper. Shape the potato into balls of 250 g each, ready to cook the roestis in individual blini pans. Heat 10 ml. clarified butter in each of four blini pans and add a ball of potato to each one. Press down on the potato with a palette knife to cover the surface of the pans. Run the palette knife around the side of the pans to prevent the roesti from sticking. Cook the roestis for 6–7 minutes over a gentle heat until crisp and golden brown. Turn the roestis out onto a plate. Wipe out the pans and add another 10 ml. clarified butter. Cook the other side of the roestis until crisp and golden brown. Transfer the roestis to the oven and cook for a further 5 minutes. The roestis can be prepared in advance and reheated in the oven before serving.