Heat gas or charcoal grill. Brush steak with 2 tablespoons dressing; sprinkle with 1/4 teaspoon of the salt and the pepper.
Cut 4 (18x12-inch) sheets of heavy-duty foil. Generously spray half of one side of each foil sheet with cooking spray. Place 1/4 of potatoes evenly in center of each sprayed portion of foil. Generously spray potatoes with cooking spray; sprinkle with remaining 1/2 teaspoon salt. Fold unsprayed half of foil loosely over potatoes so edges meet; seal edges with tight 1/2-inch folds and fold again.
When grill is heated, place packets on gas grill over medium-high heat or on charcoal grill over medium-high coals; cover grill. Cook packets 10 minutes, turning over several times. Place steak on grill with packets; cook 10 to 15 minutes longer, turning steak over once or twice and turning packets several times, until steak is desired doneness and potatoes are tender.
In large bowl, toss cooked potatoes, cheese and 2/3 cup dressing. Cut steak across grain into thin slices. Divide potato mixture evenly onto serving plates; top each with steak slices and sprinkle with onions.