Sprinkle the ribs with salt and pepper and place them meaty-side up on an aluminum foil-lined rimmed baking sheet or shallow baking dish. Roast for 30 minutes and then drain off any accumulated fat from the baking sheet. Turn the ribs over and increase the oven temperature to 400 degrees F. Return them to the oven for an additional 30 minutes.
While the ribs cook, make the glaze.
Drain the aji panca and place it in a blender along with the tamarind paste, brown sugar, Worcestershire sauce, lemon juice, orange juice and garlic. Puree to make a thick paste, scraping the jar as necessary. Set aside 1/4 of the glaze for serving.
After the ribs have cooked at 400 degrees F for 30 minutes, begin basting them with the glaze every 15 minutes until the ribs are tender, about 1 hour, turning the ribs meaty side up again after 30 minutes.
Remove the ribs from the oven, brush with the reserved glaze and serve.
