Position the rib slabs on a large wire rack sitting on a rimmed baking sheet (line the baking sheet with foil to catch any drips). Coat the ribs on both sides generously with Kansas City-Style Rib Rub, about 1/3 cup each, rubbing the seasonings into the meat well. Refrigerate for up to 2 hours.
When you’re ready to cook the ribs, heat a grill to low heat so that the temperature hovers between 300- and 350-degrees F. Pour 1 ½ cups of water onto the rimmed baking sheet that the ribs are sitting on. Wrap the entire baking sheet (with rack and ribs) tightly with heavy duty aluminum foil. Transfer to the gill, close the grill lid, and cook until the ribs are fork-tender, usually 1 ½ to 2 hours. Unwrap the ribs and slather them with the Kansas City-Style BBQ Sauce. Close the lid and allow to cook until the sauce sets on the ribs, 15 to 20 minutes longer.
Cut the ribs apart and serve hot, with the slaw and additional BBQ sauce, if desired.