FOR THE PORK: Combine salt, sugar, lime zest, orange zest, garlic, cumin, oregano, and pepper flakes in bowl. Using sharp knife, trim fat cap on pork to ¼ inch. Cut 1-inch crosshatch pattern in fat cap.

Place pork on large double layer of plastic wrap. Sprinkle pork all over with salt mixture. Wrap pork tightly in plastic, place on plate, and refrigerate for at least 12 hours or up to 24 hours.

Adjust oven rack to middle position and heat oven to 325 degrees. Unwrap pork; transfer to Dutch oven, fat side up; and pour 2 cups water around pork. Cover, transfer to oven, and cook until meat registers 175 degrees in center, 2½ to 3 hours.

Uncover pork and continue to cook until meat registers 195 degrees in center and fork slips easily in and out of meat, 45 minutes to 1¾ hours longer. Transfer pork to carving board, tent with aluminum foil, and let rest for 45 minutes.

FOR THE MOJO: While pork is resting, heat oil and garlic in small saucepan over low heat, stirring often, until tiny bubbles appear and garlic is fragrant and straw-colored, 3 to 5 minutes. Let cool for at least 5 minutes. Whisk pineapple juice, orange juice, lime juice, mustard, cumin, salt, pepper, oregano, and pepper flakes into cooled garlic oil.

Slice pork as thin as possible (some meat may shred; this is OK) and transfer to serving platter. Serve with onion and mojo.