In a 10 to 12-inch saute pan, heat the olive oil over medium heat until almost smoking.
Meanwhile, season the flour with salt and pepper. Dredge the veal medallions in the seasoned flour. Add to the pan and saute, turning once, until golden brown on both sides, about 2 minutes per side. Transfer the veal to a platter and keep warm.
Add the mushrooms to the pan and saute until they are lightly browned and the juices have evaporated. Add the Marsala, bring to a boil, and reduce by one third. Add the cold butter and replace the veal medallions in the pan. Bring just to a boil, reduce the heat, and simmer for 5 to 6 minutes.
Stir in the thyme leaves and season the sauce with salt and pepper, to taste. Transfer the veal to a large platter, pour the mushroom sauce over the meat, and serve.
"Molto Italiano: Simple Italian Recipes for Cooking at Home" by Mario Batali, Harper Collins, 2005