Peel and wash the potatoes, cover and cook in salted water for about 20 minutes. Drain the beetroot and collect the liquid. Finely dice beetroot and cucumber separately.
Peel and finely dice the onions. Heat 1 tbsp. lard in a pan. Fry the onions in it. Add the corned beef, crush it coarsely and fry it crumbly while stirring for about 5 minutes.
Warm up the milk. Drain potatoes, add milk and 4 tablespoons beetroot juice. Mash everything with a potato masher. Stir in corned beef. Season with salt and pepper. Keep warm on the switched off stove.
Heat 1 tablespoon of lard in a frying pan and fry 4 fried eggs in it. Season with salt and pepper. Arrange the lobskaus with cucumber and beetroot cubes, rollmops and fried egg.