Build the marinade: Toast the dried chiles in a dry skillet (30–60 seconds per side). Soak in hot water for ~10 minutes until softened. Blend everything (chiles + soaking liquid, garlic, onion, pineapple juice, vinegar, spices, achiote) until completely smooth. You’re looking for a thick, brick-red paste—almost like a loose mole.
Slice chicken into large strips or chunks (not too small, you want caramelization). Toss thoroughly with marinade. Marinate at least 2 hours (overnight is ideal).
Preheat oven to 450°F. Use a large sheet pan (or two - crowding kills browning). Lightly oil the pan. Spread chicken in a single layer. Scatter pineapple chunks around and on top. Add a few thin onion slices to mimic traditional pastor texture.
Roast for 18–22 minutes. Switch to broil (high) for 3–5 minutes to char edges. You want crispy edges, caramelized pineapple, and slightly charred bits.
Rough chop the cooked chicken on a cutting board. Toss with pan juices. Optional: squeeze fresh lime over everything.