Using a sharp knife or a cleaver, trim away any fat or gristle from the pork. Cut the meat across the grain into slices about 1/8-inch thick. Then cut the slices, with the grain, into matchstick-size shreds about 1 1/2 inches long, and place in a bowl. Add the Marinade ingredients and toss lightly to coat. Cover with plastic wrap and let marinate for at least 1 hour in the refrigerator, or overnight. Combine the Sauce ingredients and stir to dissolve the sugar. Heat a wok or a skillet, add the oil, and heat until very hot. Add the pork shreds and stir-fry over high heat, stirring continuously, until the meat loses its raw color and separates. Remove with a handled strainer or a slotted spoon and drain in a colander. Clean out the pan. Arrange the leeks in a mound on a platter, making a slight indentation in the center for the meat. Reheat the pan, add the sauce mixture, and heat, stirring continuously, until thickened. Add the cooked meat, toss lightly to coat, and spoon over the leeks. Toss the leeks and pork lightly before spooning the mixture into the Mandarin pancakes or flour tortillas.