Combine the chiles, garlic, peppercorns, half of the onion and half of the pineapple in a medium saucepan. Add 3 cups water and bring to a boil. Remove from the heat and cover to allow the chiles to steep, 30 minutes. Puree in a blender and add the vinegar and salt. In a large resealable plastic bag place the pork and the marinade and refrigerate 8 hours or up to overnight.
Preheat a grill for cooking over medium heat. Reassemble the slices of pork back into its original roast shape and skewer it in several places to hold it together. Roast on the grill, turning frequently, until the pork is fully cooked and an instant-read thermometer inserted in the center registers 180 degrees F, 35 to 40 minutes. (You can also cook slices of pork directly on the grill until the pork is fully cooked, 2 to 3 minutes per side.) Remove from the heat and let rest for 10 minutes.
Combine the cilantro with the remaining onions and pineapple and mix well. Season to taste with salt.
Chop the pork into bite-sized pieces and place them in a serving bowl. Serve with warm corn tortillas and the pineapple salsa.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Recipe courtesy of Tacos El Bronco, Brooklyn, New York