Lightly pound the chicken breasts to 1/4-inch thick. Season with salt and pepper and lay a sage leaf on each breast. Lay 1 slice prosciutto over each and fold in half like a book. Secure the two sides with a toothpick and dredge whole piece in flour seasoned with salt and pepper. In a 12 to 14-inch saute pan, heat oil until just smoking. Add chicken and saute until golden brown on both sides. Add shallots and mushrooms and cook until mushrooms have sweated, about 5 to 6 minutes. Add marsala and chicken stock and cook over high heat until reduced by half. Swirl in butter and serve.