Heat the oven to 200C/180C fan/gas 6. Bring a medium pan of water to the boil. Add the potatoes and cook for 8 mins. Drain and leave to steam-dry in a colander for a few minutes. Toss the potatoes with ½ of the oil and plenty of seasoning in a baking tray. Put in the oven for 20 mins, tossing halfway through the cooking time.
Meanwhile, heat the remaining oil in a frying pan over a medium heat. Add the leek and fry for 5 mins, or until beginning to soften. Stir through the garlic for 1 min, then add the cream, capers and 75ml hot water, then bring to the boil. Stir through the chives.
Heat the grill to high. Pour the creamy leek mixture over the potatoes, then sit the salmon fillets on top. Grill for 7-8 mins, or until just cooked through. Serve topped with extra chives and capers and a salad on the side, if you like.