Preheat the oven to 300 degrees F. Line a large baking sheet with a piece of aluminum foil that is large enough to cover the pan twice (you will be folding this over the ribs and sealing it).
Arrange the ribs, meat side up, in one layer on the prepared baking sheet. In a small bowl, combine the Rib Rub, salt, pepper, and celery salt, and stir to combine. Divide the seasoning evenly between the 2 slabs of ribs, coating them well on both sides. Fold the extra length of foil over the ribs, and seal it tightly on all sides. Place the baking sheet in the oven and bake, undisturbed, for 2 1/2 to 3 hours, or until the ribs are very tender.
Preheat a grill to medium-high (or leave the oven on).
Remove the baking sheet from the oven and peel back the foil so that the ribs are exposed. Using a basting brush or the back of a spoon, coat the meaty side of the rubs lightly with barbecue sauce. Place the slabs of ribs, meaty side up, on the grill and cook until the barbecue sauce is thickened and lightly browned, 5 to 10 minutes. (Alternatively, return the baking sheet to the oven and continue to bake the ribs, without the foil covering, until the barbecue sauce is thickened and lightly browned, 20 to 30 minutes.)
Plating / Assembly:
Remove the ribs from the grill (or oven), and let them cool briefly. Then cut between the ribs and serve, with additional barbecue sauce on the side if desired.