Boil the potatoes in a large pan of lightly salted water for 10-12 mins until just tender. Drain well and leave to cool for 5 mins.
Place the oil, vinegar, mustard and sugar in a large serving bowl and whisk together until thoroughly blended. Season with salt and freshly ground black pepper.
Slice the warm potatoes and toss in the dressing with the red onion. Scatter over the snipped chives and tarragon and serve warm or cold.
Notes
Leave the skins on the potatoes for more flavour and fibre, just lightly scrub before boiling to remove any dirt.