Prepare the sauce first. Melt the butter in a saucepan and sauté the onion, leek, celery and carrot until soft. Add the flour and cook for 2 more minutes over medium heat while stirring all the time.
Add the Muscadel wine, vinegar, chicken stock and herbs, bring to a boil and reduce to a simmer. Cover with a lid. Simmer 10 – 15 minutes or until the carrots are very soft. Puree everything in a blender until smooth. Strain through a sieve, if preferred. Add hot water if necessary to make a sauce the consistency of thin cream.
Taste, season with salt and pepper and a few more drops of vinegar, if desired. Keep warm while the fillets are cooking. Rub the pork fillets with olive oil, salt and pepper and heat a bit of olive oil in a frying pan. Fry the fillets on al sides until a light golden brown and just cooked through. Do not overcook.
Slice the fillets against the grain or in diagonal slices, arrange on a warm serving platter and spoon the Muscadel sauce over. Poached prunes, quinces or small crab apples make an ideal accompaniment to this elegant dish.
Variation: Use pork chops or steaks with this sauce and prepare in the same way. The pork chops or steaks can be fried in a pan and roasted for 10 – 15 minutes in the oven to ensure that they are cooked through evenly. Serve with polenta wedges or crisp potato wedges.