Add 1 small red onion, cut into wafer-thin slices, to this salad when you pour over the dressing.
Cook the potatoes in a pan of boiling salted water for 12-15 mins until just tender. Meanwhile, in a small bowl, whisk the mustard with the lemon zest and juice, and slowly whisk in the oil to form a smooth emulsion. Season to taste.
Drain the potatoes and tip into a large bowl. Drizzle over the dressing and gently mix. Leave to stand for 15 mins to allow the potatoes to absorb the flavours. To serve, stir the parsley and spring onions into the potatoes. Season to taste and serve immediately.