In a blender, combine pineapple chunks, guajillo chilies, garlic, achiote paste, cumin, oregano, salt, and pepper. Blend until you have a smooth marinade.
Place the sliced pork in a bowl and cover it with the marinade. Ensure each piece is well-coated. Marinate for at least 2 hours, or preferably overnight in the refrigerator.
If using a vertical rotisserie (trompo), stack the marinated pork and cook it on the rotating spit for 1.5 to 2 hours until fully cooked and the outer layer is crispy.
If using an oven or grill, cook the marinated pork at a low temperature, turning occasionally for even cooking.
Warm the corn tortillas.
Place a few slices of the cooked pork on each tortilla.
Garnish with fresh cilantro, diced onions, and pineapple chunks. Add salsa if desired.
Serve the Tacos al Pastor warm and enjoy the burst of flavors in each bite.