Boil the potatoes in salted water for 10-12 minutes or until just tender. Drain and mix gently in a large bowl with the sliced red onion. Season to taste and cool slightly.
Add oil and vinegar dressing to potatoes and refrigerate for one hour or until chilled. Meanwhile, chop the apple and mix with the lemon juice. Mix into the potato along with the sausage and gherkins.
Beat the mayonnaise, mustard, sour cream, sugar, poppy seeds and just enough milk to make a dressing the consistency of cream. Stir this carefully into the salad, spoon into a serving dish and sprinkle with the chopped herbs.
To make the Oil and Vinegar Salad Dressing: Whisk together all ingredients. Store refrigerated in sealed jar. Whisk again just before serving. This will make more than you need, but it is a nice dressing to have ready to use on a variety of salads.