Lightly roast the hazelnut flakes in a pan without fat, remove and let them cool down. Peel garlic and press it through a garlic press. Peel, wash and thinly slice the potatoes. Grease a large gratin dish and place the potatoes in the dish in flakes. Season with salt and pepper, spread garlic and thyme on top and sprinkle with hazelnut leaves and parmesan.
Add milk and cream. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 45 minutes. In the meantime peel and halve the onions and cut them into fine strips. Drain the chanterelles. Heat the oil in a pan. Tie the fillet steaks with kitchen string and fry them in the hot oil for approx. 6 minutes, turning them over. Season with salt and pepper, remove from the pan, wrap in aluminium foil and keep warm. Put the fat in the hot pan and fry the onions for about 2 minutes while turning. Add chanterelles and fry everything for about 3 minutes. Deglaze with beef stock and cook again for about 1 minute.
Season with salt and pepper, remove from the pan, wrap in aluminium foil and keep warm. Put the fat in the hot pan and fry the onions for about 2 minutes while turning. Add chanterelles and fry everything for about 3 minutes. Deglaze with beef stock and cook again for about 1 minute. Season with salt and pepper. Cut 4 portions out of the finished potato gratin with a round cookie cutter (approx. 9 cm Ø) and put them on preheated plates. Arrange the fillet steaks and the chanterelle and onion melt and serve immediately garnished with thyme