Peel the cucumber, cut in half and core with a spoon. Cut the cucumber halves into strips. Peel and dice the onions, sauté in butter or margarine until translucent. Add the cucumber to the onions, sauté briefly. Dust with flour, sweat the flour a little, then deglaze with the stock and crème fraîche. Stir in the mustard and season with salt, pepper and sugar.
Wash the fish fillet, cut into strips, season with salt, pepper and lemon juice and cover with the cucumber and cook for 4 minutes.
Pluck the dill from the coarse stalks, chop and mix into the ragout.
Boiled potatoes, mashed potatoes or rice go well with it.