Dice the bacon, leave out. Add the butter and oil, fry the salted and peppered fillets until golden brown. Add the peeled onions and sauté.
Pour the calvados over the fillets, flambé for 20-30 seconds. Add the leek, thyme, parsley and mushrooms cut into rings. Pour in the cider and simmer for 10-15 minutes. Finally stir in the crème fraiche and reduce a little.