Mix well brown sugar, paprika, chili powder, garlic and onion powder, mustard, black pepper, cayenne pepper, celery seeds, and ground thyme in a small bowl.
Pat the ribs dry with paper towels to remove excess moisture.
Remove the membrane on the rib's backside by sliding your fingers under the thin membrane, loosening it, and pulling it off.
Rub mustard all over the ribs and generously season both sides with dry rub.
Place the ribs directly on the smoker, meat-side up, and smoke for three hours, spritzing with water or apple juice as needed.
Lay out two a couple of large sheets of aluminum foil, each big enough to wrap one rack of ribs.
After 3 hours, carefully transfer the ribs with an oven mitt to the prepared foil paper. Or use a foil pan and cover it tightly to seal in the moisture so the ribs don't dry out.
Place the wrapped ribs back on the smoker and continue cooking, undisturbed, for approximately 2 hours.
Carefully remove the ribs from the smoker with an oven mitt and place them on a clean, hard surface. Open the foil slowly, watching out for the hot steam. At this point, the ribs should be tender.
Brush both sides of racks of the ribs with BBQ sauce (check out my super easy homemade BBQ sauce). Gently remove the foil and return the ribs to the smoker.
Smoke for an hour to get a nice, sticky sauce layer. If ribs are browning quickly, remove them from the smoker. You don't want burnt ribs.
Let them cool slightly and serve with pickled jalapenos, pickled onions, baked beans, coleslaw, or potato salad. Whatever rocks your boat! Enjoy!