Trim excess fat from steak; cut into thin, bite-sized slices. In a 3 1/2 to 4-quart slow-cooker, combine soy sauce, brown sugar, ginger and garlic. Stir in meat. Cover; cook on the LOW setting for 7 to 9 hours or on the HIGH setting for 3 to 4 hours.
Use a slotted spoon to remove meat from juices. Pour juices into a measuring cup; skim fat. Measure 1 1/2 cups juices (add water if necessary); place in a saucepan. Stir together cornstarch and the 2 tablespoons water; add to saucepan. Cook and stir until thickened and bubbly. Stir in the cooked meat. Cook and stir for 2 minutes more.
Meanwhile, brush rolls lightly with margarine. Place, cut side up, on the unheated rack of a broiler pan. Broil 4 to 5 inches from heat for 2 to 3 minutes or until lightly toasted.
Top bottom halves of the toasted rolls with meat mixture and desired toppings. Cover with roll tops.