In a small bowl, whisk salt, sugar, chile powder and pepper. Sprinkle 1 tbsp. of the spice rub on the meaty side of each rib rack. Let stand for 1 hour or wrap and refrigerate for up to 1 day. (Let chilled ribs stand for 1 hour before baking.)
Position racks in the top and bottom thirds of the oven; preheat to 300 degrees . Line 2 baking sheets with foil. Lay 2 rib racks, meaty side up, on each sheet. Bake until fork-tender and browned, 2 to 2 1/2 hours.
Meanwhile, in a medium pan, heat the EVOO over medium. Add the onion; cook, stirring, for 5 minutes. Add the garlic; cook, stirring, for 2 minutes. Stir in 1/2 cup water, the apple jelly, ketchup and vinegar. Bring to a boil. Reduce the heat to low. Simmer, stirring, until reduced by one-fourth, 20 minutes.
Preheat broiler. Brush bottoms of ribs with the sauce, then generously brush the tops. Working in batches, broil until glaze bubbles. Cut between ribs. Serve with the remaining sauce.