SPRAY a 10-inch skillet with the cooking spray and heat over medium-high heat for 1 minute. Add the mushrooms and onion and cook until tender. Remove the vegetables and set aside. Remove the skillet from the heat.
SPRAY the skillet with the cooking spray. Add the beef and cook until it's well browned and the juices evaporate, stirring often.
STIR the soup, water, mustard and vegetables into the skillet and cook until the mixture is hot. Toss the rice with the parsley, if desired. Serve the beef with the rice.
*To make slicing easier, freeze beef for 1 hour.