In a large saucepan boil potatoes and caraway seeds in salted water until they are easily pricked with a knife. Drain and rinse under cold water. Peel potatoes while they are still warm and slice.
In a jar with a lid combine mustard, sugar, pickle jar liquid, vinegar, oil, salt and a generous amount of pepper. Add onion, close the jar and shake well.
Place potato slices, gherkins and apples in alternating layers in a salad bowl and drizzle each layer with the dressing. Cover and let marinate in the fridge for at least 4 hours. Before serving add chives and mix thoroughly.