In a bowl, whisk the mustard with all the other ingredients up to and including the quark, season, set aside 4 tbsp of the dressing. Mix the celeriac and apple into the remaining dressing, leave to infuse for approx. 30 mins. Plate up the apple & celeriac salad together with the lamb's lettuce, drizzle the reserved dressing over the lamb's lettuce. Sprinkle with pumpkin seeds.