Soak the onion slices in a bowl of ice water. Cover the potatoes with water in a pot; add a pinch of salt, cover and bring to a boil. Reduce to a simmer and cook, uncovered, until tender, 8 to 10 minutes. Drain and let cool slightly; pat dry.
Meanwhile, whisk the creme fraeche, vinegar, olive oil and mustard in a large bowl to make a smooth dressing. Stir in the dill. Drain the onion, pat dry and add to the dressing. Add the potatoes and cucumber, season with salt and pepper and toss to coat.
Divide the watercress among 4 plates and add some potato salad to each. Flake the trout with your fingers and scatter on top. Serve with pumpernickel bread and/or pickled beets, if desired.