Trim the pork shoulder or loin off any excess fat. Cut the meat into thin slices, about ¼ inch thick.
Season the pork with salt on both sides.
Place the meat slices in a shallow dish, and pour the marinade over them, ensuring that each slice is coated evenly.
Cover the dish with a lid or plastic wrap, and let the meat marinate in the refrigerator for at least 4 hours, or ideally overnight.
Before you're ready to start cooking the meat, grill the pineapple on a skillet. Preferably a griddle that achieves that delicious char taste.
Chop the pineapples in small cubes to get them ready for the tacos.
Cook. Heat a griddle or cast iron skillet to medium-high heat and add two tablespoons of oil. Cook the meat slices until they are nicely browned and cooked through. About 3 minutes per side.
Cook in separate batches so that you don't overcrowd the skillet. Note: Clean the skillet between batches to remove the burnt marinade. Add a little more oil between each batch of meat.
Transfer the meat to a dish as it cooks. When the meat is all cooked, cut into thin small strips and return to the dish. Set aside.
In same skillet, add a tablespoon of oil and heat over medium-high heat. Add the sliced onions and saute until soft. Add the sliced meat back in the skillet and combine with the onions.