First of all you got to pour 3/4 cup of the pineapple juice, vegetable oil, achiote paste, adobo sauce, chipotles, garlic and salt in a food processor. Once all the mentioned ingredients are included in the processor, let it mix. Use the pineapple juice mixture to mix it along with the pork and move around to coat it. Let the pork marinate in the fridge for about 1 to 2 hours.
The next thing you’ll need to do is to preheat an iron cast grill or skilled to medium heat. Don’t bother about keeping the heat to medium at all times, if it increases slightly it doesn’t matter. But don’t let it increase too much. Lightly oil the grill and add tortillas. Toast them about 30 seconds on each side. Remove the tortillas after heating them, and store them in a way that they don’t cool down. Use a hot-pot if you must.
Once the tortillas are done, raise the heat of the grill to high and add another spoon of vegetable oil on it. Remove the stored pork from the refrigerator, wiping off any excessive mixture over it. Place the pork over the grill and let it cook thoroughly.
Remove the pork from the grill and chop it into small bite-sized pieces. Take half of the onions and some fresh pineapple along with them and cook them till the turn brown, which would take about 2 minutes. Add the remaining pineapple juice and the chopped pork back to the grill to cook for about a minute.
Remove the pork-juice mixture from the grill and place them in the tortillas. Don’t overfill the tortillas as they would be hard to manage while eating. Top it with the remaining onions, cilantro, cotija and salsa. Your Tacos Al Pastor are finally ready to be savored.