One day in advance: Boil the potatoes in a large pan of salted water. Peel when still warm. Cool and keep covered in the refrigerator.
To prepare: Grate the potatoes and season with salt.
Heat the butter in a frying pan. Add the grated potatoes to the pan - use a spatula to spread the potato in to a cake. Spread the milk over the potatoes and reduce the heat. Sprinkle the cheese on top and cover the pan with a well-fitting lid or a large plate. Fry for 30 minutes until a crisp brown crust has formed.
Invert the rosti cake on the lid or plate.
Make the dressing for the salad leaves by whisking together the olive oil, mustard, white wine vinegar and seasoning.
Toss the leaves through this dressing and serve with the rosti.
«This dish can be prepared with left over boiled potatoes. Peeled potatoes should not be kept in the refrigerator for more than a day. Unpeeled potatoes will keep for 2-3 days, but they will be harder to peel.»