Peel potatoes, grate or cut into thin strips and mix with the salt.
Cut onions into very fine onion rings.
Melt the butter and lard (or oil) in a frying pan. Add the diced bacon, onions 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.