8 to 10 metal skewers; a chimney starter; a smoker box; apple or cherry wood chips soaked in water for 30 minutes
Combine adobo, ancho powder, oil, achiote paste, brown sugar, salt, cumin, garlic and 1/4 cup water in a small saucepan. Whisk until uniform while bringing to a simmer to bloom all the spices and flavors, then simmer, about 5 minutes. Take off heat to cool completely, 15 to 20 minutes.
Coat all slices of pork with marinade. Spray a plastic quart container with cooking spray. Stuff the container with the slices of marinated pork, arranging them in a stack. Cover and place in the fridge 8 hours or overnight to marinate.
Slide marinated meat gently out of quart container. Cut the pineapple into 4 equal rounds. Reserve 2 whole slices of pineapple and core the remaining 2 slices, then cut them into chunks. Place the stack of marinated meat in between the reserved slices of pineapple. Evenly skewer the stack with 8 to 10 metal skewers to make the "pastor".
Heat a grill for cooking at about 275 degrees F. Using a chimney starter, ignite 3 to 4 charcoal briquettes. Place the briquettes in a smoker box with apple or cherry wood chips that have soaked in water 30 minutes, and place the box directly on the grate.
Place onion and pineapple chunks in a small drip tray or roasting pan. Position the pan right on the grill, and the pastor on the elevated grill shelf directly above it. Place smoker box directly in front of the drip pan, so that some of the heat and smoke from the wood can reach the pastor.
Cook for 4 hours, rotating a quarter every 45 minutes to 1 hour. Take it off the heat and let rest for at least 1 hour.
Heat a cast-iron skillet over medium-high heat. Dice onions and pineapple from drip tray and sear in the skillet until caramelized, about 5 minutes.
Holding the pastor vertical, shave off shards of pork. Sear off in the skillet until caramelized, about 5 minutes.
Serve with corn tortillas, salsa verde and fresh cilantro.