Preheat the oven to 200ºC. Scrub the potatoes and dry them with paper towel. Thread them onto metal skewers – this helps them to cook more quickly. Brush the skin of the potatoes with 1 tbsp (15ml) of the oil, then sprinkle with a little salt. Arrange on a baking tray and bake for 1 – 1¼ hours or until tender.
Remove the potatoes from the skewers and cut them in half lengthways. Scoop out the flesh, leaving a layer of potato next to the skin about 1 cm thick. (Use the scooped-out flesh for fish cakes or mash to make a savoury pie topping.) Cut each piece in half lengthways again, and place, flesh-side up, on a large, clean baking tray.
Melt the margarine with the remaining oil and season with salt and pepper to taste. Lightly brush this mixture over the flesh side of the potato skins. Return to the oven and bake for a further 12–15 minutes or until golden and crisp.
Meanwhile, cut the smoked salmon into thin strips and sprinkle with the lemon juice. Mix together the yogurt, capers and chopped dill in a bowl, then stir in the salmon.
Allow the potato skins to cool for 1–2 minutes, then top each with a little of the salmon and yogurt mixture. Garnish each with a small sprig of dill, then serve while the potato skins are still warm.