1. Pull off and discard the membrane from the back of the ribs, then combine the mustard, garlic powder, pepper, sugar, paprika, and salt to make a spice rub. Coat the rib slabs generously with oil and sprinkle with the spice rub, gently rubbing it in with your fingers. Wrap the baby back ribs in plastic and chill in the fridge for four hours or overnight.
2. Prepare the oven rack to sit at the middle shelf, heat it to 275F, and then bring the ribs from the fridge and unwrap. Next, line a baking pan with foil and position a wire rack on it. Then, wrap the rib slabs with a foil cover, place them on the rack and cook for three hours.
3. Remove the baking pan, unwrap the ribs and discard the foil. Then, set the ribs on the rack and coat each top with half the BBQ sauce. Toss back in the oven for 45 more minutes to one hour, by which the meat should be pull-apart tender. The internal temperature should also read 145F.
4. Take the ribs out and cover all surfaces with the rest of the BBQ sauce. Then, broil for three to five minutes, or until you notice the sauce caramelize and bubble. Finally, remove the tray, and rest the ribs for five minutes before serving.

If you need more information, take a peek at this video.