For the dressing, stir together the mustard, vinegar and oil, season. Depending on the size, cut the potatoes crossways in half or into thirds, cover and simmer gently in a pan of salted water for approx. 15 mins. Drain and leave to cool slightly.
Slice the radish, chop the spring onions into rings, add to the dressing along with the potatoes, mix together.
Cut the mushrooms into pieces. Heat the butter in a non-stick frying pan, sweat the mushrooms for approx. 3 mins., season, leave to cool slightly, mix in with the salad.