Pick out the larger potatoes and halve them, making them roughly the same size as the smaller ones. Put all the potatoes into a pan of boiling salted water. Boil for 15 to 20 minutes, or until the potatoes are just cooked, and drain in a colander.
Put the lemon zest and half the lemon juice into a bowl and add the vinegar. Normally, when making a dressing, I stick to one type of acid, but in this case using vinegar and lemon juice together gives the dish a lovely zinginess. Pour in three times as much olive oil as vinegar, and add the capers. Season the dressing with salt and pepper. Mix everything well, then add the warm potatoes and toss around until they are all well coated.
Finely grate the horseradish into a bowl – be confident with the amount you use as you need the heat to go with the salmon and potatoes – and mix it into the crème fraîche with the remaining lemon juice and some salt and pepper. Sprinkle most of the dill or fennel over the cooked potatoes and toss again.
Lay your smoked salmon out on a big plate or platter. Don’t be too neat – I want you to make it look rustic! Just pinch it up here and there so that it looks wavy and pile the dressed potatoes on top. Dollop over the horseradish crème fraîche, drizzle with some olive oil and sprinkle over the rest of the dill or fennel. Served with a glass of wine and some nice bread, this makes a delicious lunch.