Mix yoghurt, mustard and honey. Season to taste with salt, pepper and lemon juice. Cover and put aside at room temperature. Peel and finely chop the onions. Peel cucumber, cut in half lengthwise and scrape out the seeds with a spoon.
Then again halve lengthwise and cut into large cubes. Wash the tomatoes and dill. Halve the tomatoes if necessary. Chop the dill roughly. Heat butter in a flat pot. Sauté onions in it until golden brown.
Add cucumber cubes and sauté for about 2 minutes. Season with salt and pepper. Add vermouth and wine and bring to the boil. Cover and simmer for 7-8 minutes. Add tomatoes after 4 minutes. Meanwhile wash the fish fillet, dab dry.
Cut into 8-10 pieces. Season with salt and turn in flour. Heat oil in a coated pan. Fry the fish for about 3 minutes on each side until golden brown. Mix dill with the cucumber and season to taste.
Arrange the fish on top and spread the yoghurt and mustard sauce over it.