Wash an peel the potatoes and place them in a large pot with enough water to cover them. Bring to a boil over high heat and then reduce the heat to medium-low. Cook the potatoes until they are fork-tender, but not falling apart, about 30 minutes. Cook time depends on the size of potatoes. For small potatoes 15 minutes is enough. Drain and let cool for a few minutes.
Chop the red onion into small dices. Cut also the gherkin in small dices.
Wash and cut the apple (if using) into bite-size pieces. Add the apple, gherkin and red onion in a bowl.
Whisk together the mayonnaise, sour cream, sweet mustard, black pepper, salt, and sugar in a small bowl until well combined.
Once the potatoes have cooled, cut them into bite-size pieces and add them in the bowl.
Pour the dressing over the mixture and stir until everything is well coated. Taste and adjust the seasoning as needed.
Cover the bowl with plastic wrap and refrigerate for at least 1 hour before serving to allow the flavors to meld together.
Decorate with chopped spring onion or chives.