Combine 2 tablespoons of the sugar, the salt and spices in a small bowl. With a small, sharp knife, loosen the membrane from the underside of each rib rack. Use your fingers or a pair of pliers to pull off the membrane from the rack. (Alternatively, leave the membrane on the rack but carefully score it with a knife.)
Rub 1 tablespoon of seasoning mix into each side of each rack. Place the ribs in a large roasting pan, cover with plastic wrap, and let marinate in the fridge for at least 6 hours. (The ribs can be stored this way for up to 1 day.)
Heat the oven to 325°F. In a glass or ceramic bowl, whisk together 1/4 cup of the cider, the remaining 1/4 cup sugar, the apple butter, brandy, vinegar and mustard. Cover and refrigerate until ready to use.
Remove the ribs from the refrigerator and lift them up so you can scatter the onion slices under them. Pour the remaining 1 cup apple cider in the pan and arrange the ribs meat side down on the onions. Cover the pan with foil and roast the ribs until the meat is tender and begins to pull away from the bones, about 2 hours. Uncover; allow to cool at least 30 minutes and up to 2 hours.
Heat a grill to medium-high. Grill the ribs until heated through and slightly charred, about 5 minutes per side. Brush the ribs on all sides with the basting sauce, then grill the ribs about 3 minutes longer per side. Transfer the racks to a cutting board and cut between the bones to separate the ribs. Arrange on a platter and present any remaining sauce on the side.