Since I couldn`t find a recipe for the Sylt salad dressing, I mixed it myself and the result is very similar to the original.
Cut the onion as small as possible, press through the garlic and mix both with yoghurt, mustard, sour cream, olive oil, vinegar and dill and whip the sauce well. If the sauce is too thick, dilute with milk as required. Season to taste with salt, pepper, sugar and lemon juice.