Drain the mushrooms. Wash and clean the zucchini and cut them into slices about 3 mm thick. Put potatoes with mushrooms, courgette and smoked salmon in an ovenproof dish (approx. 1 litre capacity). Wash chives, dab dry and cut into small rolls.
Stir cream, stock and sauce thickener until smooth. Season with salt and pepper. Add the chives, except for a little bit for sprinkling, and pour over the potato and salmon gratin. In a preheated oven (electric cooker: 200 °C/ gas: level 3)
Bake for about 20 minutes. Sprinkle with the remaining chives