Peel, wash and cut the potatoes into pieces. Cook in boiling salted water for about 20 minutes. Wash fish, dab dry, cut into pieces, salt and sprinkle with lemon juice.
Peel the cucumber and cut in half lengthwise. Remove seeds and cut the flesh into thick slices. Wash and halve the tomatoes. Heat oil in a pan. Fry the fish for about 4 minutes and take it out.
Fry the cucumber in the frying fat for about 3 minutes. Add stock, bring to the boil. Cook cucumber for another 3 minutes. Add cream, bring to the boil and stir in the sauce thickener. Season with salt and pepper. Add fish and tomatoes and warm up in the sauce.
Wash the dill, chop finely and add to the fish-cucumber ragout. Arrange potatoes and ragout on plates and serve.