Trim the ribs: remove the papery membrane from the bone side, tidy any ragged edges, and pat dry.
Make the rub: in a bowl combine light brown sugar, kosher salt, paprika, black pepper, garlic powder, onion powder, chili powder, mustard powder, and cayenne pepper.
Lightly coat both sides of each rack with yellow mustard, then sprinkle the rub evenly over all surfaces; let sit at room temperature 30–60 minutes while you preheat the smoker.
Prepare the smoker to 225°F with clean-burning smoke; add 4 hardwood chunks (hickory or apple) to your heat source per manufacturer’s guidance.
Mix the spritz: combine apple juice, apple cider vinegar, and water in a spray bottle; reserve about 0.5 cup of this mixture in a cup for wrapping later.
Place ribs in the smoker bone side down, leaving space between racks; close the lid and maintain 225–250°F.
Smoke for 3 hours, spritzing lightly after the first 90 minutes and every 45 minutes thereafter; look for a deep mahogany color and set bark that doesn’t wipe off when touched.
Wrap: place each rack on a sheet of heavy-duty foil or unlined butcher paper; pour about 0.25 cup of the reserved spritz mixture under each rack, seal tightly, and return to the smoker meat side up.
Continue cooking wrapped for 1.5–2 hours, until the ribs are very tender: a probe slides in with little resistance (around 195–203°F), the rack bends easily, and 0.25–0.5 inch of bone is exposed.
Make the sauce while ribs are wrapped: in a saucepan combine ketchup, molasses, granulated sugar, Worcestershire sauce, distilled white vinegar, finely grated yellow onion, and minced garlic; simmer over low heat, stirring, until glossy and slightly thickened, 10–15 minutes. Remove from heat and keep warm.
Unwrap the ribs and return them to the smoker unwrapped, meat side up. Brush a thin layer of sauce over the tops and cook 20–30 minutes to set the glaze until tacky, brushing once more if desired in the last 10 minutes.
Rest the ribs on a board 10 minutes, then slice between the bones. Serve with additional warm sauce on the side.