I’m going to assume you don’t own a vertical rotisserie, so my method relies on direct grilling.
Step 1: If you are using guajillo chiles, place them in a bowl with 4 cups of warm water and let them soak for an hour.
Step 2: Using a very sharp knife, cut the pork loin across the grain into thin slices. Arrange half the onion slices in a large mixing bowl or roasting pan. Arrange the pork over the onion slices, then place the remaining onion slices on top.
Step 3: Drain the soaked chiles, if using, setting aside 1 cup of soaking liquid. Tear open each chile and scrape out and discard the seeds. Coarsely chop the chiles and place them in a blender with the vinegar and the reserved 1 cup of chile soaking liquid, then puree until smooth. If you are using chile powder, place it and 3/4 cup of water in the blender along with the vinegar. Add the achiote paste, if using, and the tomatillos, garlic, bay leaves, salt, pepper, cinnamon, oregano, cumin, aniseed, and thyme and process to a smooth pourable puree. Alternatively, you can puree the ingredients in a food processor, but in this case, puree the solid ingredients first, then add the liquids.
Step 4: Pour the marinade over the pork and onions, turning them to coat both sides. They don’t have to stay in layers. Let the pork marinate in the refrigerator, covered, for 3 to 6 hours. The longer it marinates, the richer the flavor will be.
Step 5: Set up the grill for direct grilling and preheat it to high.
Step 6: When ready to cook, brush and oil the grill grate. Drain the pork and onion slices, discarding the marinade. Place the pineapple slices on the hot grate and grill until golden brown, 3 to 5 minutes per side. Keep the grilled pineapple warm on the side of the grill or on a platter. Place the pork and onion slices on the grate and grill until golden brown, 2 to 4 minutes per side.
Step 7: Transfer the pork, onion, and pineapple to a cutting board, stacking several slices of pork with a slice of onion and pineapple. Thinly slice them crosswise.
Step 8: Warm the tortillas on the grill for about 30 seconds per side. Place some sliced pork, onion, and pineapple on a tortilla. Top with cilantro and salsa, if desired, to taste. Fold the tortilla in half and eat.
Serve with the rest of Steven’s Cinco de Mayo grilled feast.