How to Make It
Preheat the oven to 325°. Line 2 large rimmed baking sheets with foil and set the ribs on them, meaty side up. In a mortar, crush the fennel seeds with the thyme, crushed red pepper, rosemary, 1 1/2 tablespoons salt and 2 teaspoons black pepper. Rub the spice mix all over the ribs and roast for about 2 hours, until the meat is tender.
Meanwhile, in a medium saucepan, heat the olive oil. Add the onion, garlic and a generous pinch of salt and cook over moderately high heat, stirring, until the onion is softened, 3 to 5 minutes. Add both vinegars along with the ketchup and brown sugar and bring to a boil. Simmer over moderate heat, stirring frequently, until the sauce is thick and reduced to 2 cups, about 15 minutes.
Remove the ribs from the oven and turn on the broiler. Brush the underside of the racks with some of the sauce. Broil 1 sheet of ribs 8 inches from the heat until browned. Flip the ribs and repeat on the other side. Move the ribs to the bottom rack of the oven to keep warm while you glaze the rest.
Transfer the racks to a work surface. Cut in between the bones to form individual ribs and mound on a platter. Pass the remaining sauce at the table.