Heat grill to medium heat.
Cook and stir half-and-half, 1/3 cup A.1., 1/4 cup cheese and flour in small saucepan on medium heat 4 to 5 min. or until cheese is melted and mixture comes to boil.
Grill steak for 5 to 6 min. on each side or until medium doneness (160°F), brushing occasionally with remaining A.1. Remove steak from grill; let stand 5 min.
Cut steak across the grain into thin slices; place on serving plate. Drizzle with cheese sauce. Sprinkle with remaining cheese and onions.