Halve the potatoes (keep small ones whole) and place in a large saucepan. Cover with cold water, bring to the boil and simmer for 18-20 minutes, or until just tender. Drain well and set aside to cool.
Meanwhile, mix the mayonnaise with the wholegrain mustard and season well. Once the potatoes have cooled, toss with the mustard mayo, shallots and spring onions.