Cook the potatoes, make the dressing
Gently boil the whole potatoes with skin in salted water until tender. To make the dressing, whisk together mustard, crème fraîche, olive oil, lemon juice, pepper and salt in a bowl, until you get a silky emulsion.
Drain the cooked potatoes well and slice in half. In a large bowl, dress the potatoes with crème fraîche emulsion. Add a handful of chopped olives and sun dried tomatoes, capers, most of the dill / fennel, sliced red onion and toss everything together well.
Flake the smoked trout or salmon over the top. Sprinkle with thinly sliced red chili pepper and the rest of the fresh dill / fennel. Drizzle with a bit of olive oil and grate some lemon zest on the top for an extra kick of juiciness.