Peel, wash and roughly dice the potatoes and celery. Cook in salted water covered with lemon for about 20 minutes
Clean the chanterelles, wash them if necessary. Peel and chop the onion. Thyme wash, pluck. Wash the fillet, dab dry. Fry all around in 2 tablespoons of hot oil. Season and fry for about 15 minutes. Wrap in aluminium foil, let rest
Heat 1 tablespoon of oil in the frying fat. Sauté the onion in it. Add chanterelles and fry over high heat for about 3 minutes. Season with thyme, salt and pepper. Deglaze with Calvados and cream. Bring to the boil, thicken and season to taste
Drain the potatoes and celery. Add butter, mash and season. Cut the fillet open. Stir meat juice into the mushroom sauce. Serve everything
Drink: cool rosé wine