Wash the potatoes, peel them in half and cover with cold water. Cucumbers wash, peel, halve lengthwise and remove seeds. Cut the cucumbers into pieces. Wash and peel carrots and cut them into thick slices.
Onions peel, 1 halve and the 2. finely throw dice. Wash the dill, shake dry and cut off the stems. Put 375 ml water, white wine, 2-3 tbsp lemon juice, carrot and onion halves, dill stems, some salt and pepper in a casserole dish.
Bring to the boil, cover and simmer at low heat for 8-10 minutes. Cut plaice fillets in half lengthwise and sprinkle with a little salt. Cover each fillet with about 1 slice of salmon and roll up.
Pin it with wooden skewers. Cover the potatoes and cook in boiling salted water for about 20 minutes. Heat oil in a large pot, fry diced onions in it until they start to brown. Add cucumbers and season with salt and pepper.
Add 150 ml water, bring to the boil and simmer covered for 10-12 minutes. Remove vegetable pieces and dill stalks from the wine stock with a skimmer. Place the plaice rolls in the stock, cover and simmer over a low heat for 8-10 minutes.
Cut the dill tips finely. Pour the braised cucumbers onto a sieve, collect the vegetable water and put it back into the pot. Bring to the boil. Mix starch and approx. 3 tbsp. cold water, stir into the vegetable water and simmer for approx. 1 minute.
Add sour cream and dill, except for a little bit for sprinkling, heat and season with salt, pepper and about 1 tablespoon lemon juice. Fold in cucumbers. Drain potatoes. Remove the plaice rolls from the stock with a skimmer, drain and arrange on plates with the cucumber vegetables and potatoes.
Sprinkle with remaining dill.