In a food processor, combine jalapeño, garlic, onions, spices, oregano, tomatoes, pineapple and pineapple juice.
Pour over pork (you’ll use about 3/4 of the amount of sauce you use), making sure you coat the pork on all sides. Set the remainder of the sauce aside – you might need it later.
Marinade pork overnight in the fridge.
Preheat oven to 200 degrees C and place pork on a roasting dish covered with foil.
Cook for 1 hour at 200 degrees C and then reduce oven temperature to 125 degrees C.
Cook for 8-10 hours until the pork starts to fall apart. Remove pork from the slow cooker and shred the meat gently with two forks, spooning some of the cooking sauce over it.
At this point, you can refrigerate the meat. When you are ready to eat it, reheat slowly in a saucepan, adding some of the reserved sauce.
Serve on corn tortillas with condiments of your choice.