Cover potatoes with skin and cook in boiling water for about 20 minutes. Then quench, peel and let cool.
Finely dice the bacon. Peel and finely dice the onion. Heat butter in a pot. Fry half of the onion cubes in it. Dust with 1 tbsp. flour and sauté until light brown. Stir in milk and cream. Bring to the boil and simmer for about 5 minutes.
Stir the whole mustard into the sauce. Season with salt, pepper and 1 pinch of sugar and keep warm.
Cut the potatoes into slices. Heat 2 tablespoons of oil in a large frying pan and fry the potatoes and bacon for 10-15 minutes while turning them until crispy.
Meanwhile, wash the fish, dab dry, cut into larger pieces and turn in 4 tablespoons of flour. Heat 1-2 tablespoons of oil in a second pan. Fry the fish in it for about 3 minutes on each side until golden brown.
Add the remaining onion to the potatoes and fry briefly. Season with salt and pepper. Serve the fried potatoes with fish and sauce. Cucumber salad goes well with it.