Clean the leeks, wash them and cut them with the tender green into fine rings. Wash and peel the potatoes and cut them into thin slices with a cucumber slicer.
Layer the leeks and potatoes in an ovenproof dish, seasoning each with salt and pepper.
Pour the cream on the side. Wash the marjoram, toss dry and finely chop the leaves.
Peel and finely dice the apples. Mix the marjoram, apples, cheese and sunflower seeds and season with pepper. Spread the apple mixture on the gratin and top with the butter in flakes.
Put the dish on the rack in the oven (center).
Cook at 200 degrees for about 40 min. until the potatoes are firm to the bite and the surface is browned.