Cucumbers wash, peel and halve lengthwise. Remove seeds with a spoon and cut the halves into thick rings. Peel and finely chop the onion. Heat the fat in a pan and fry the onion in it.
Add cucumbers, also briefly sauté and season with a little salt and pepper. Pour in broth, bring to the boil, cover and stew for 5-6 minutes. In the meantime, boil 750 ml water and 1 teaspoon of salt for the mashed potatoes.
Take the pot from the stove, add cold milk and stir in the puree flakes with a wooden spoon. After 1-2 minutes stir the puree again and keep covered warm. Wash the dill, dab dry and chop finely, except for something to garnish.
Add cream and mustard to the braised cucumbers, add sauce thickener while stirring and bring to the boil again briefly. Stir in dill, except for a little bit for sprinkling, and season with salt, pepper and sugar. Cut salmon into large cubes and spread on the cucumber pan.
Sprinkle with dill and garnish. Add mashed potatoes extra.