Cook the onion and garlic until lightly charred. Toast the guajillo and ancho chiles in a cast iron pan until fragrant, then remove the stems and seeds from the chiles.
Blend the toasted chiles, onion, garlic, Spanglish Asadero Al Pastor seasoning, pineapple juice, orange juice, white vinegar, and achiote paste until smooth.
Tenderize the pork belly skin, butterfly the belly, score the meat side, and coat generously with the marinade.
Roll tightly, tie with butcher’s twine, salt the skin, and refrigerate uncovered overnight.
Bank the charcoal to the back of the grill, place a drip pan underneath, and set the rotisserie for 300°F indirect heat. Mount the pork belly on the spit with two large pineapple chunks on each end. If the skin starts getting too dark, add a heat deflector.
Cook 2½ hours, checking regularly, until the pork reaches 180°F and the pineapple is beautifully charred.
Remove the pork and pineapple, crank up the heat, then return the pork to the rotisserie with the lid open until the skin puffs into crispy chicharrón. Rest 30 minutes while you mash together the guacamole ingredients.
Dice the charred pineapple, slice the pork into ½-inch slices with a bread knife, then top each slice with guacamole, pineapple, onion, cilantro, and a squeeze of fresh lime.