Cucumbers wash, clean and cut into fine slices. Peel and halve red onions and cut into strips. Clean, wash and finely dice the bell peppers. Mix vinegar with 2 tablespoons of water, salt, pepper and sugar. Fold in 2 tablespoons of oil. Mix cucumber, paprika and about 3/4 of the onions with the marinade, let it stand for at least 30 minutes, season again.
Peel and wash the potatoes and cook in boiling salted water for about 20 minutes. Wash fish, dab dry, cut in half crosswise, season with salt and pepper. Heat 1 tablespoon of oil in a large, coated pan. Fry the fillets on both sides for 3-4 minutes at medium heat. When turning, add the remaining onions and fry them. Wash the chervil, dab dry and set aside a little for garnishing. Remove the coarse stems from the remaining chervil and chop the leaves finely. Melt the fat in a saucepan, dust with flour, sauté and gradually add stock and milk. Simmer for 1-2 minutes while stirring. Season to taste with salt, pepper and some sugar. Stir in about 3/4 of the chervil. Keep the sauce warm.
Melt the fat in a saucepan, dust with flour, sauté and gradually add stock and milk. Simmer for 1-2 minutes while stirring. Season to taste with salt, pepper and some sugar. Stir in about 3/4 of the chervil. Keep the sauce warm. Drain the potatoes, let them steam briefly on the switched off hotplate and swivel them through with the remaining chervil. Arrange fish, sauce and potatoes on preheated plates, garnish with chervil. Add cucumber salad