Puree 3/4 cup of the pineapple juice, the vegetable oil, achiote paste, adobo sauce, chipotles, garlic and salt in a food processor. Mix the pineapple juice mixture with the pork in a freezer bag and move around to coat. Marinate the pork in the fridge, 1 to 2 hours.
Preheat a cast-iron skillet or grill to medium-high heat. Lightly oil the skillet and add the tortillas, toasting, about 30 seconds per side. Remove the tortillas and store in a towel to keep warm.
Raise the heat under the skillet to high and add 1 tablespoon vegetable oil. Remove the pork from the bag and wipe off any excess pineapple juice mixture. Cook the pork in batches, until charred and cooked through.
Remove the pork from the skillet. Chop the meat into bite-size pieces. Add half of the onions and the fresh pineapple and quickly cook until lightly browned, about 2 minutes. Add the remaining 1/4 cup pineapple juice and the chopped pork back to the skillet with the juices.
Place the pork, pineapple and onion mixture in the tortillas. Top with the remaining onions, cilantro, cotija and salsa. Order a margarita, apply SPF 2 and take a bite.