Boil the potatoes with skin in salted water. Then peel and cut into thin slices while still warm.
Peel the onion and cut into small cubes. Bring to a boil with beef broth, vinegar and mustard.
Pour the broth over the potatoes and stir gently until the potatoes have absorbed the liquid. Season with salt, pepper and olive oil.
Cut the wild garlic into coarse strips and the radishes into fine slices and stir into the lukewarm potato salad.
Tear the trout fillets into coarse pieces and arrange on the potato and wild garlic salad.