Peel and cut the potatoes into slices and boil in lightly salted water until cooked. Germans would traditionally boil them with the skins on then peel and slice them when cooked.
Dice the onion, peel, core and dice the apple, cut the gherkins into small cubes, brown the bacon and drain off the fat and finely chop the parsley.
Pour the stock into a small saucepan. Add the vinegar, oil, sugar, mustard and season well with salt and pepper. Stir to combine well and bring to the boil. Allow to boil for about 30 seconds then remove from the heat.
Combine potato slices, diced onion, gherkin cubes, diced apple, and bacon cubes in a bowl. Pour over the dressing and stir through. Allow the salad to sit and soak up the dressing for about 2 hours, stirring regularly. Add the parsley and check the seasoning and stir through again before serving.