The chiles. If using chile pods, toast the flat pieces a few at a time in an ungreased skillet over medium heat, using a metal spatula to press them firmly against the hot surface until they release their aroma and change color slightly, then flip them over and press to toast the other side. Collect in a bowl, cover with hot tap water and let rehydrate until pliable, 20 to 30 minutes. Drain, reserving the soaking liquid. Place the chile in a blender jar. If using powdered chile, measure the powdered chile into a blender jar. Add 1 ¼ cups boiling water, blend to mix and let stand 20 minutes.
Finish the marinade and marinate the meat. While the chile (whole or powdered) is rehydrating, in an ungreased skillet over medium heat, roast the garlic, turning regularly until blotchy black in places and soft. Cool slightly, peel and add to the blender with the chile. Add the cinnamon, black pepper, cumin, oregano, vinegar and 2 teaspoons of salt. Blend until smooth, adding a little soaking liquid or water if necessary to keep everything moving through the blades. Remove 1/2 of the red adobo mixture from the blender jar and store for a later use (adobo can last for 6 months in a closed container in the refrigerator). Then add achiote (if using), agave and oil to the blender jar. Finally, add 1 piece of pineapple to the blender jar and blend until very smooth. The marinade should be about as thick as ketchup. Spread it on both sides of the meat. If possible, cover and refrigerate for several hours or overnight.
Grill or saute the meat. For grilling: Heat a gas grill to quite hot or build or charcoal fire and let the coals burn until covered with white ash but still quite hot. Brush or spray the pineapple with oil and grill until nicely caramelized and warm. Chop into small pieces, scoop onto a rimmed baking sheet and keep warm in a very low oven. Grill the meat: I like to grill it on a hot grill almost exclusively on one side to replicate the one-sided sear you get on the vertical spit of tacos al pastor. Chop the meat into small pieces, scoop onto baking sheet with the pineapple and keep warm. For sauteing: Heat a very large (12-inch) heavy skillet (preferably cast iron) over medium-high. Brush or spray the pineapple with oil and sear in the skillet until nicely caramelized and warm. Chop into small pieces, scoop onto a rimmed baking sheet and keep warm in a very low oven. Add a light coating of oil to the pan, then, in batches, sear the meat in a single layer (I like to cook it almost exclusively on one side to replicate the one-sided sear you get on the vertical spit of tacos al pastor). As the meat is cooked, chop it into small pieces and add to the baking sheet with the pineapple.
Serve. Mix together the onions and cilantro. Make delicious tacos on the warm tortillas with meat, pineapple, onions and cilantro and a spoonful of salsa.