If necessary, remove the thin membrane from the back of each rack of ribs; discard the membrane. Combine the paprika, brown sugar, kosher salt, cayenne pepper, garlic salt and black pepper in a small bowl. Rub the meat with the oil and then rub liberally with the spice blend. Leave at room temperature for 45 minutes.
Meanwhile, set the grill for indirect cooking at 250 degrees F, with wood chips for flavor if using. (I use bourbon barrel staves. Cherry and hickory would be good, but stay away from mesquite.) Wrap the soaked chips in a double layer of heavy-duty aluminum foil. Place the foil-wrapped chips on the coals.
Add the spareribs, meat-side up. Close the lid and cook, flipping occasionally, until the meat is smoky and just tender, about 2 1/2 hours. Brush the spareribs with the Sweet Heat Barbecue Sauce and continue cooking until the sauce is nicely caramelized and the meat is completely tender, about an additional 30 minutes. Serve with Sweet Heat Barbecue Sauce on the side.
In a saucepan, heat the oil over medium heat; add the onions and simmer until soft and melted, 5 to 7 minutes. Add the ketchup, vinegar, Worcestershire sauce, mustard, black pepper, brown sugar, cayenne pepper and lemon juice. Bring to a boil, decrease the heat to low and simmer until the flavors have smoothed and mellowed, at least 10 and up to 30 minutes. Taste and adjust for seasoning with salt and pepper.
Store in an airtight container in the refrigerator. It will last for months.