Wash the potatoes and cook in boiling water for about 20 minutes. Drain and rinse with cold water. Peel and slice the potatoes. Peel onion and chop finely. Heat the broth, add onions and mustard seeds, pour over the potatoes and leave to stand for about 30 minutes.
In the meantime, wash the cucumber, quarter it lengthwise and cut into thin slices. Clean and wash the radishes and also cut them into slices. Wash chives, dab dry and cut into small rolls.
Mix oil, vinegar and mustard. Season with salt and pepper. Mix cucumber, radish and marinade with the potatoes. Wash the fish and dab dry. Halve the fillets. Season with salt and pepper, turn in flour and fry in hot fat until golden brown.
Cut the bacon into cubes. Add to the fish and fry until crispy. Add capers. Arrange two fillets each with some lukewarm potato salad on a plate. Garnish with parsley as desired.