In a bowl combine al pastor seasoning packet with lime juice and vegetable oil. Add pork and pineapple and toss until well coated. Cover and refrigerate to marinate for a minimum of 30 minutes or up to overnight.

In another bowl combine onions and cilantro. Toss until well mixed. Refrigerate until needed

Preheat grill to 400-450°F.

Thread pork and pineapple onto soaked skewers. Grill about 5-6 minutes per side until pork has reached an internal temperature of 145°F with a 3-minute rest.

Allow pork to rest 5-10 minutes before removing from skewers. Roughly chop pork and pineapple together.

To serve, place a few slices of avocado on top of a tostado if using. Top with a ¼ cup of cooked pork and pineapple mixture. Finish with about a teaspoon of onion mixture. Serve with lime wedges if desired.