PLACE pineapple and onions on a cookie sheet, brush with oil on all sides and season with kosher salt. Roast until cooked through, softened and lightly browned. Let cool.
DICE pineapple and onions into ½" pieces and mix together. Reserve.
PASTOR MARINADE
REMOVE stems and seeds from chiles, place them in a small sauce pan and cover with water, bring to a simmer, then let cool.
REMOVE chiles from liquid and place in a blender with all other ingredients and purée until smooth.
PORK PASTOR
SEASON pork on both sides with kosher salt and rub liberally with pastor marinade.
REFRIGERATE overnight.
HEAT a barbecue grill on high. Make sure the grill is clean and wipe the it with vegetable oil. Grill pork on each side for about 3 minutes. Let cool and dice into ½ inch cubes. Reserve.
TO FINISH
HEAT a large cast iron skillet over medium high heat. Add oil and in batches sear the pork pastor.
ADD remaining marinade and reserve hot.
HEAT each tortilla on both sides and place 2 ounces of pork in the center of each tortilla. Garnish each taco with 1 tablespoon pineapple mixture, sprinkle with cotija cheese and cilantro. Enjoy.