1. First prepare the red sauce in the following manner: In a heavy 10- to 12- inch skillet, heat the vegetable oil over moderate heat. Add the onions and garlic and, stirring frequently, cook for about 5 minutes, or until they are soft and translucent but not brown.
2. Stir in the tomatoes and their liquid, the tomato puree, chilies, mustard, sugar, vinegar and salt, and bring to a boil over high heat. Cook briskly, uncovered, until the sauce is thick enough to hold its shape almost solidly in the spoon.
3. Remove the pan from the heat and taste the red sauce for seasoning. Set it aside.To barbecue the ribs, light a 2-inch-thick layer of charcoal briquettes in a charcoal grill equipped with a rotating spit. Let the coals burn until white ash appears on the surface.
4. Thread the spareribs on the spit, running the spit through the meat over and under alternate pairs of ribs. Then secure them at both ends with the sliding prongs.
5. Fit the spit into place about 6 inches above the surface of the coals and barbecue the ribs for 45 minutes, or until they are lightly and evenly browned. Watch carefully for any sign of burning and regulate the height of the spit accordingly.
6. With a pastry brush, spread the red sauce evenly on both sides of the spareribs. Basting the ribs every 5 minutes or so, continue to barbecue them for about 30 minutes longer, or until they are richly colored and glazed with sauce.
7. To serve, remove the spit from the grill, unscrew the prongs and slide the spareribs off the spit onto a heated platter. Before serving, insert skewers into each section of ribs if you like.
