Heat oven to 350°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray. Arrange potatoes evenly in dish.
In large bowl, mix barbecue sauce with pork and water; pour mixture evenly over potatoes. Cover tightly with foil.
Bake 40 to 50 minutes or until potatoes are tender and mixture is hot and bubbly.
Uncover dish; sprinkle cheese and onions over top. Bake uncovered 10 to 15 minutes longer or until cheese is melted. Top individual servings with potato topper.