Clean and dry the potatoes, then brush them all around with olive oil using a pastry brush. Sprinkle a little pepper and coarse sea salt on the potatoes, then wrap them individually in aluminum foil.
Roast the potatoes in the oven for 1 hour until tender. They are well done when you can easily poke them with a skewer or knife. Let the potatoes cool slightly before serving.
Meanwhile, make the sauce by mixing the sour cream and mustard.
Finely chop the dill and grate the lemon. Mix this into the sauce and season with salt and pepper.
Serve the jacket potato with the sauce and the hot smoked salmon.