There are many, many kinds of rosti recipes and they can be made from raw potatoes, cooked potatoes and with or with meat and cheese. Very versatile dish. The potatoes are best if cooked the day before.
Peel potatoes, grate or cut into thin strips and mix with the salt.
Heat the butter and lard (or oil) in a frying pan. Add the diced bacon and potato and fry gently, stirring gently and frequently. If you want a slimmer version, you can pre-cook the bacon and this way omit the bacon fat.
Bring together into a cake and cover with an upside-down plate. Fry over gentle heat for 20 minutes.
Pour the milk over the top and fry, covered, for about another 10 minutes. Tip out onto the plate and serve.