Prepare the chiles.
Heat a skillet over medium heat and toast the dried chiles for about 30 seconds per side, until fragrant. Place in a large bowl and cover with hot water. Let soak for 15 minutes until softened.
Blend the marinade.
Drain the chiles and add them to a blender with pineapple juice, vinegar, garlic, achiote paste, Mexican oregano, cumin, smoked paprika, oil, salt, and pepper. Blend until smooth.
Marinate the pork.
Place the thinly sliced pork shoulder in a large bowl. Pour the marinade over the pork and toss to coat. Cover and refrigerate for at least 4 hours, preferably overnight for maximum flavor.
Layering the pork.
If using skewers or a loaf pan, layer the pork slices with onion and pineapple. This mimics the vertical spit style. If grilling or pan-searing, keep the slices separate.
Cook the meat.
Grill: Cook pork slices over medium-high heat for 4–5 minutes per side until slightly charred.
Oven: Roast layered pork stack at 375°F for 1 hour, then broil until edges are crisp.
Skillet: Cook slices in batches over medium-high heat until browned and cooked through.
Slice the pork.
Once cooked, slice the pork thinly against the grain. Mix with chopped grilled pineapple for that signature sweet-savory combo.
Assemble the tacos.
Warm corn tortillas, then fill with pork, grilled pineapple, diced onion, and chopped cilantro. Finish with lime juice.