To make marinade: In a large bowl, combine soy sauce, mirin, dark brown sugar, sesame oil, garlic cloves, scallions, ginger, apple and set aside.
Place the pork tenderloin on a cutting board and cover with plastic wrap. Using a metal mallet, pound the tenderloin until it is about ¼ inch thick. Once you are done, place in marinade and refrigerate for 24 hours.
Next day: Heat oil in a sauté pan to a medium-high heat. Place pork in pan and heat on each side for about 2 to 3 minutes. Remove from pan and cut into small cubes. Return cut pork to pan, add the remaining marinade and cook on high heat for about 8-10 minutes, or until marinade reduces by half. Set aside. Then, in a bowl, mix together the pineapple and cilantro. Set aside.
To assemble: Place pork mixture in center of tortilla, top with pineapple and garnish with spicy mayo to taste.