Mix the mustard with all the other ingredients up to and including the stock, stir in the shallot. Heat the oil in a non-stick frying pan. Stir fry the leek for approx. 4 mins., remove and leave to cool slightly. Combine the leek with the vinaigrette, season, wipe out the pan. Peel the warm potatoes, cut in half lengthwise (depending on size) and then cut crosswise into slices approx. 5 mm thick. Arrange the potatoes on plates and fan them out, season with salt. Drizzle with the leek vinaigrette.
Heat the oil in the same pan. Reduce the heat a little. Fry the fish for approx. 4 mins. on each side, season. Grate over the lemon zest, drizzle a little lemon juice over the salmon and serve with the potato salad.