In a saucepan with boiling salted water add the potatoes and cook for 10 minutes until firm to the bite. Drain and rinse with cold water. Set aside and let cool for about 10 minutes.
Dissolve stock cube in the 200ml boiling water and pour into a big salad bowl. Add red onion and blanch for a few minutes. Stir in mustard, honey and white wine vinegar. Add potatoes and gherkins and carefully mix until well combined. Add salt and pepper to taste and cover with cling film. Chill in the fridge for about 1 hour.
Take out of the fridge and taste. Season with salt, pepper or vinegar as needed. Stir in parsley and serve.