Clean the chanterelles, rinse and pat dry. If necessary cut smaller. Clean and wash the cabbage and cut into fine strips. Peel and chop onion. Cut bacon into strips
For the mashed potatoes, peel and wash the potatoes, cover and cook in salted water for about 20 minutes. Peel the onion
Dab the fillet dry and season with salt and pepper. Sauté in 1 tablespoon of hot oil all around and take out. Fry the bacon in the frying fat until crispy. Add chanterelles and fry for approx. 5 minutes. Season with salt and pepper and take out. Heat 1 tbsp. oil in the frying fat. Fry the onion cubes in it. Deglaze with wine and cream, bring to the boil. Simmer for about 5 minutes. Thicken sauce with sauce thickener. Stir in mustard. Add chanterelles and bacon to the sauce. Season to taste
Sauté the cabbage strips in 1 teaspoon butter while turning. Season with salt and pepper and add approx. 1/8 litre of water. Steam covered for about 3 minutes. Pour off
Cut the fillet into slices about 3 cm thick. Spread the cabbage on an ovenproof deeper plate or in a flat casserole dish. Place the fillet slices on top. Spread chanterelle sauce over it. Bake in a preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for approx. 15 minutes
For the mashed potatoes, dice onion and fry until crispy in 1 tsp. hot butter. Drain the potatoes. Warm the milk and add it to the potatoes. Mash everything finely. Season with salt and nutmeg. Sprinkle mashed potatoes with the onions. Serve with fillet au gratin