Peel the cucumbers and cut them in half lengthwise. Scratch out the seeds with a spoon. Cut the cucumbers into thicker slices. Peel and chop the onion
Heat the fat in a large pan (with lid). Sauté the onion in it. Steam cucumbers briefly. Add stock and 400 ml water, bring to the boil. Cover and stew for 5-10 minutes.
Bring 3/4 l salted water to the boil. Wash and finely chop the dill. Dice salmon. Remove salted water from the stove. Pour in milk and stir in puree. Stir sour cream and mustard into the cucumber pan
Bind cucumbers with sauce thickener and season with salt and pepper. Stir the dill and sprinkle the salmon cubes over it. Add mashed potatoes