In a small bowl, mix together the chili powder, paprika, cumin and 1 tablespoon salt.
Lay 2 half-racks of ribs flat in a 9-by-13-inch glass baking dish. Repeat with the remaining 2 half-racks. Using your hands, coat each rack with the spice rub. Set aside.
For the sauce: Heat the oil in a medium saucepan over medium-high heat. Add the onions and 2 teaspoons salt and cook until translucent and lightly browned, 5 to 7 minutes. Add the garlic and chiles and cook for 2 minutes. Add the apple cider vinegar, and cook, scraping the bottom of the pan with a wooden spoon, for 3 minutes. Add the mustard and preserves. Stir to combine. Bring to a boil, reduce to a simmer and cook to allow the flavors to marry, about 3 minutes. Turn off the heat and carefully pour half the sauce over the ribs. Pour 1/2 cup of the chicken stock into the bottom of each dish. Cover the baking dishes with foil and place into the oven on the middle rack. Bake until the meat is tender, about 1 1/2 hours.
Meanwhile, cook the remaining sauce over medium-high heat until reduced by half, about 20 minutes.
Heat a grill pan or a gas or charcoal grill over medium-high heat.
Using a pastry brush, glaze the baked ribs with the reduced sauce. Place the ribs meat-side down onto the hot grill and cook until slightly charred, 3 to 5 minutes. Turn the ribs over and cook for an additional 5 minutes. Flip one final time, allowing the ribs to cook until perfectly charred, 3 to 5 minutes more. Remove from the grill and rest for 10 minutes before cutting.