Preheat the oven's broiler, and set the oven rack about 3 inches from the heat source.
Trim fat from steak. Place steak on cold broiler pan; broil for 5 minutes. Turn, and broil until desired degree of doneness - 3 to 7 additional minutes for rare, 8 to 10 additional minutes for medium.
Make the sauce while the steak is cooking. Coat a small saucepan with cooking spray, and add mushrooms and onion; cook and stir until tender. Stir in Marsala wine, water, parsley, beef base, and pepper. Simmer, uncovered, until liquid is reduced to 3/4 cup, about 4 minutes.