Heat oil in a 10-inch cast-iron skillet over medium-high. Sprinkle steak with salt and pepper. Add steak to pan; cook 4 minutes on each side for medium-rare or until desired degree of doneness. Place steak on a cutting board; let stand 10 minutes. Add mushrooms, shallots, and garlic to pan; cook 6 minutes or until mushrooms are browned. Add brandy; cook 1 minute or until liquid evaporates, stirring constantly. Stir in stock, mustard, and Worcestershire sauce; cook 2 minutes. Remove pan from heat; stir in half-and-half. Cut steak across the grain into thin slices. Divide steak between 2 plates; top evenly with mushroom mixture and parsley.