Mix the mustard with all the other ingredients up to and including the chives, season.
Place a steaming basket into a wide pan, fill with water until it is just below the bottom of the basket, bring to the boil and reduce the heat. Add the potatoes, season with salt, cover and cook for approx. 15 mins. over a medium heat until soft. Allow the potatoes to cool slightly and arrange on plates with the tomatoes. Heat the clarified butter in a frying pan. Reduce the heat a little, fry the mushrooms in batches for approx. 3 mins., season. Arrange the mushrooms on top of the salad and drizzle with the dressing.