Wash the potatoes and cook in salted water for about 20-25 minutes. Drain potatoes, let cool and peel. Peel and finely dice the onion. Cook onion in the broth for 3-4 minutes. Cut potatoes into slices and pour the stock over them, mix carefully and let it cool down.
Mix salad mayonnaise with lemon juice and ketchup, season with salt and pepper and fold into the potatoes. Rinse the cucumber, peel it in strips and cut it into slices. Dab the cucumber slices dry and mix into the potatoes together with the cranberries.
Prepare fish fingers in the oven without fat according to the instructions on the packet.