How to Make It
In a small bowl, combine the brown sugar, salt, mustard, fennel, black pepper, cayenne and paprika. On 2 large rimmed baking sheets, sprinkle the spice mix all over the ribs, pressing and patting it. Cover with foil and refrigerate overnight.
Preheat the oven to 250°. Pour off any liquid on the baking sheets, cover the ribs with foil and roast for about 3 hours, until the meat is tender but not falling off the bone. Pour off any liquid on the baking sheets.
Meanwhile, in a saucepan, melt the butter. Add the onion, garlic and thyme and cook over moderate heat until the onion is softened, about 5 minutes. Add the ketchup, vinegar, beef broth, hot sauce, Worcestershire sauce and molasses and bring to a boil. Simmer over low heat, stirring occasionally, until thickened, about 30 minutes.
Preheat the broiler and position a rack 10 inches from the heat. Brush the ribs liberally with the barbecue sauce and broil for about 10 minutes, turning and brushing occasionally with the sauce, until well-browned and crispy in spots. Transfer the ribs to a work surface and let rest for 5 minutes. Cut in between the bones and mound the ribs on a platter. Pass any extra barbecue sauce on the side.