Boil the potatoes with the skin on, until just past al-dente.
Drain and place in a medium sized bowl.
When the potatoes have cooled slightly but are still warm, gently mix in the tomatoes, olives, onions and parsley.
Whisk together olive oil, balsamic vinegar, Dijon mustard, garlic, salt and pepper and pour over the potato mixture and mix well.
Add a little more olive oil and vinegar before serving if it looks dry.Serve at room temperature.