Season the chicken with salt and pepper. Rinse the mushrooms and pat dry. Season the flour with salt and pepper and dredge the chicken breasts in the flour mixture and set aside. Heat oil in a skillet over medium high heat and saute the chicken until it is nicely browned on one side, about 4 minutes. Add the garlic. Turn the chicken breasts and scatter the mushrooms over them. Continue frying, shaking the skillet and stirring the mushrooms. Cook for about 4 minutes, and then add the vinegar, chicken stock, red pepper, rosemary, onion and thyme. Cover tightly and simmer over medium low heat for 10 minutes, turning occasionally. Transfer the chicken to a warm serving platter and cover with foil. Set aside. Continue simmering the sauce, uncovered, over medium high heat for about 5 to 7 minutes. Add the butter and stir well. Pour this mushroom sauce mixture over the chicken and serve. Makes 6 servings.