Cover the potatoes and boil them in water for about 20 minutes. Quench, peel and let cool down.
Meanwhile wash the meat and pat dry. Heat 2 tablespoons of oil in a frying pan. Fry the meat for 4-6 minutes on each side. Season with salt and pepper. Let it cool down.
Drain the tuna. Rinse anchovies. Puree both with mustard, mayonnaise and sour cream. Season to taste with salt, pepper and lemon juice. Stir in capers.
Peel and finely chop the onion. Heat 1 tablespoon of oil in the frying oil, fry the onion until transparent. Add 150 ml water with broth and vinegar. Bring to the boil and simmer for 2-3 minutes. Cut potatoes into slices in the hot stock, mix and let them simmer for about 10 minutes.
Fold in tuna mayonnaise and leave to stand for 30-40 minutes.
Clean, wash and finely dice the peppers. Wash cucumber, peel if necessary and slice or cut into thin slices. Halve fillets lengthwise and cut into slices. Fold everything into the potato salad.
Season to taste with salt, pepper and sugar.