2) Add the remaining 1.5 tsp of oil to the skillet. Add the shiitakes and cook over a medium-low heat, until soft, about 4 minutes. Add the shallots, garlic and salt and cook until translucent, about 2 minutes. Pull the skillet from the heat, add the Cognac, and use a wooden spoon to scrape up any brown bits that cling to the pan.
3) Return the pan to the heat and cook until the liquid is reduced, about 1 minute. (If you are cooking over a gas flame the alcohol may flame, simply cook until the flames die down.) Add the chicken stock, and cook until saucy, about 2 minutes more.
4) Remove from the heat, stir in the sour cream and parsley, and season with salt and pepper. Spoon the mushrooms and sauce over the beef medallions and serve.
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