Place your potatoes in a pan of cold water, bring to the boil and simmer until cooked. Drain and allow to cool. In a bowl, mix the crème fraîche with the lemon zest and juice. Add the potatoes, onion, parsley and capers, then snip your cress into the bowl. Season with salt and pepper and mix well. Divide between four serving bowls and flake the smoked trout over the top.