Cook the potatoes in a large pan of salted water
until just tender, about 10-15 mins. Meanwhile,
whisk together the olive oil, mustard and lemon
juice in a large bowl. Drain the potatoes and leave
to cool for 5 mins, then tip into the bowl along
with the spring onions. Toss everything until
the potatoes are well coated. Leave to cool.
The dish will keep in a covered container in the fridge
for up to 2 days.