Place venison on a flat surface. Cut into 2-inch chunks. Place chunks, a few at a time, between wax paper sheets and lightly pound with a mallet or heavy skillet until meat is about ¼ inch thick. Season meat with salt and pepper.
Melt butter over medium heat in a large skillet. Add onion, and cook until translucent, about 4 to 5 minutes. Add venison, and cook on both sides until rare. Remove venison from pan, and set aside.
Remove pan from heat and away from any open flame or heat source. In a small bowl, combine Worcestershire sauce, brandy, stock and mustard, and stir in pan. Return pan to the heat, add mushrooms, and cook until mushrooms are soft and just cooked. Stir in heavy cream, and bring to a boil, stirring to transform sauce into a smooth mixture. Cook until sauce thickens. Season to taste with salt and pepper.
Return meat to pan to warm, about 1 minute. Spoon meat and sauce onto plates, and top with fresh parsley.