1) Sprinkle the veal with salt and pepper. Melt a third of the butter and 1 tablespoon of oil in a heavy large pan over medium-high heat. Add four veal cutlets and cook until golden brown, about 1 1/2 minutes on each side. Transfer the veal to a plate. Add another third of the butter and 1 tablespoon of oil to the pan, if necessary. Repeat with the remaining four cutlets. Set the cutlets aside.
2) Add 1 tablespoon of oil to the pan. Add the shallot and garlic. Saute until fragrant, about 30 seconds. Add another tablespoon of the olive oil, if necessary. Add the mushrooms and saute until tender and the juices evaporate, about 3 minutes. Season with salt. Add the Marsala. Simmer until the Marsala reduces by half, about 2 minutes. Add the stock and the rosemary leaves. Simmer until reduced by half, about 4 minutes.
3) Return the veal to the pan. Pour in all of the pan juices. Cook until just heated through, turning to coat, about 1 minute. Stir the remaining butter into the sauce. Season the sauce with salt and pepper, to taste.
Using tongs, transfer the veal to plates. Spoon the sauce over the veal and serve.