Heat the butter in a skillet over medium and saute the mushrooms until tender and nicely colored, then set aside. Dredge the veal in the seasoned flour, shaking off the excess. Heat the oil sufficiently to saute the veal in the skillet. Add veal and saute for 1 minute per side. Remove to a platter. Cover with foil to keep warm. Add Marsala to veal pan. Deglaze, then add chicken stock to the pan and let reduce and thicken. If the sauce does not thicken, add a mixture of 2 tablespoons of cornstarch and 2 tablespoons of cold water, stirring until it thickens. When the sauce has begun to thicken, add the mushrooms and stir to combine. Arrange the veal on a serving platter and pour the sauce and mushrooms over all. Serve immediately.