In an oven proof frying pan, heat some oil over medium high heat. Salt and pepper the veal medallions and brown them for 2 full minutes on each side, turning them only once. Finish them in the oven for 5 minutes or until the internal temperature registers 140 degrees F/60 degrees C on a meat thermometer. Transfer medallions to a plate, tent with foil and keep warm on the back of the stove.
In the same frying pan, saute the mushrooms and shallot in the butter over medium high heat for about 5 minutes or until any liquid evaporates. Deglaze the pan with the veal stock, making sure to scrape the bottom of the pan with a wooden spoon to free up the caramelized juices from the veal. Allow the liquid to reduce by half.
Reduce the heat and add the cheese stirring constantly until the sauce has a velvety consistency. Taste for seasoning, add salt and pepper if desired. Pour the sauce over the medallions and serve immediately.