Rub the potatoes and put them in a saucepan. Add the bay leaf and one sprig of mint, then cover with cold water.
Add a little salt and bring to the boil, then simmer for 15 minutes. Drain, discard the mint and bay-leaf. Keep warm in the saucepan under a lid.
Meanwhile, slice the onions diagonally, grate the lemon zest and squeeze its juice.
Dry-fry the coriander seeds, then mix with the oil, the lemon zest and juice.
Cut the cooked potatoes in half, put them in a serving dish.Season with salt and pepper, then pour the dressing over them. Toss with the spring onion and watercress.
Garnish with mint leaves and some lemon zest.