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 degrees. 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.

The roasted ribs and barbecue sauce can be refrigerated separately for up to 4 days. Return to room temperature and broil just before serving.