Wash brussels sprouts thoroughly, and remove discolored leaves. Using a sharp knife, cut off stem ends, and cut a shallow X in bottom of each sprout. Bring water to a boil in a medium saucepan; add brussels sprouts. Cover, reduce heat, and simmer 12 minutes or until brussels sprouts are tender. Drain and keep warm.
Melt margarine in a small heavy saucepan over low heat; add flour, stirring until smooth. Cook, stirring constantly, 1 minute. Gradually add milk; cook over medium heat, stirring constantly, until mixture is thickened and bubbly. Add cheese and mustard, stirring until cheese melts. Spoon sauce over warm brussels sprouts.