Clean the mushrooms, wash if necessary (drain chanterelles well from the jar). Cut the mushrooms into slices. Peel and finely dice the onion. Wash and pluck the thyme. Wash the fillets, dab dry.
Heat the oil in a large frying pan. Sauté the fillets in it all around for about 8 minutes. Season with salt and pepper. Take them out and let them rest for a while. Sauté mushrooms, onion and thyme, except for a little bit for garnishing, in hot frying fat. Season with salt and pepper
Pour 1/8 l water and cream into a small pot. Stir in sauce powder. Boil up slowly while stirring. Whip butter (only 75 g!) in small pieces bit by bit at low heat
Cut the fillets into slices. Place them in a fan shape in a greased gratin dish (approx. 30 cm Ø). Stir the mushrooms into the hollandaise sauce and season to taste again. Pour over the fillets. Bake in the preheated oven (electric cooker: 225 °C/circulating air: 200 °C/gas: level 4) for about 15 minutes. Sprinkle with remaining thyme. Goes well with Rösti corners