Preheat oven to 375 °F. Ina small bowl, combine the sherry and chicken stock. In a 10-inch non-stick fry pan, brown the onions, garlic, and Mushrooms by addint 1 tablespoon of the combined liquid, stirring until the pan dries, then repeating with another tablespoon of liquid. The pan will be slightly dry. Season the vegetables with salt and pepper. Transfer the vegetables to a plate and set the pan aside. Pound the Chicken breasts to ¼-inch thickness. Ina small bowl, combine the yogurt, sour cream, and paprika. Reheat the 10-inch pan, adding 2 tablespoons of the combined liquid. Lay the Chicken pieces in the pan and cook on medium-high heat for 2 minutes. Turn the Chicken over, add the remaining 2 tablespoons of liquid to the pan, and cook the Chicken for a final 2 or 3 minutes. Reduce hea