Preheat the oven to 400ºF. Rinse the pork rib racks under cold water and pat dry with paper towels. In a small bowl, combine the granulated onion or onion powder, paprika, garlic powder, ¼ cup brown sugar, Chinese five spice, cumin, coriander, salt, pepper and whisk until well combined. Wearing gloves for food safety, rub a light layer of the spice mix all over the ribs, massaging the spices into the meat. Wrap the pork rib racks individually in plastic wrap, making sure each rack is completely covered. Wrap the ribs again in aluminum foil, tenting the foil slightly before sealing tight. Place the wrapped racks in a single layer on rimmed baking trays. Bake at 400ºF for 30 minutes then reduce the temperature to 300ºF and continue to bake the ribs for another hour and a half. Remove from the oven, unwrap and allow the ribs to rest at room temperature for 2 hours. If not immediately glazing, refrigerate the ribs until you’re ready.
To make the glaze, combine the guava jam, hoisin, rice vinegar, shoyu, ¼ cup brown sugar, Sriracha (optional), minced garlic and grated ginger in a small sauce pan and simmer over medium heat for 15 minutes, whisking frequently, until the guava jam is completely dissolved.
To reheat the ribs, set the oven temperature to 450ºF. Next, cut the racks into individual ribs or keep the rack whole. Place the ribs on a roasting rack over an aluminum foil lined baking sheet and bake until the ribs are hot and begin to brown around the edges, about 10 minutes. Brush the ribs with a light layer of guava hoisin glaze and bake for 5 minutes more. Remove from the oven and brush once more with glaze. Serve immediately.