Mix the potatoes, leeks, mushrooms and half the Gruyère, transfer to the prepared tin Season the cream, pour over the top. Sprinkle with the rest of the Gruyère.
Gratinate for approx. 1 hr. in the centre of an oven preheated to 180°C.
Mix the vinegar with all the other ingredients up to and including the pepper. Toss the leaf salad in the dressing shortly before serving, plate up. Sprinkle with pumpkin seeds. Plate up the potato gratin with the leek and porcini mushrooms. Serve with the salad.