- Toasting the garlic chiles. Set a heavy ungreased skillet or griddle over medium heat. Roast the garlic cloves, turning frequently, until blackened in spots and very soft, 10 to 15 minutes. Cool, peel, roughly chop and place in a blender.
While the garlic is roasting, tear the chiles into flat pieces and toast them a few at a time: Use a metal spatula to press them firmly against the hot surface for a few seconds, until they blister, crackle and change color, then flip them over and press them flat to toast the other side. Break the chiles into a small bowl, cover with hot water, weight with a plate to keep them submerged and let the chiles rehydrate for 30 minutes. Drain and add to the blender.
- Finishing the marinade. In a mortar or spice grinder, pulverize the cinnamon, cloves and peppercorns. Add to the chiles along with the salt, vinegar and 3 tablespoons water. Blend until smooth. Don’t add water unless absolutely necessary or this marinade won’t do its job well. Strain the paste through a medium-mesh sieve.
- Preparing the meat. Using a sharp knife (such as a boning, filleting or slicing knife), trim off the cap of fat that covers one side of the pork loin. On the other side of the loin, there may be a strip that runs the length and has a different texture than the solid loin meat: Separate it by running your hand down the membrane that connects it to the loin, then cut it off and reserve for pork fillings. If there is a membrane or fat covering any of the surfaces, trim it off. The piece of loin should now be clear, solid meat from one end to the other.
- Cutting the meat. Lay the meat so that its length (and the grain) runs crosswise in front of you. Place one hand firmly on top of the meat, then begin slicing at one end, parallel to the work surface and 1/8 inch below the top (and, as you guessed, 1/8 inch below the level of your hand).
Work your way across the meat, but stop 1/8 inch short of the other end; do not cut through. Remove your knife, turn the meat 180 degrees and start a second cut across, ¼ inch below the top. When you’ve cut across about 1 inch, open out the top slice, bending it on the 1/8 inch “hinge” that you left at the end. Lay your hand firmly on the newly exposed top and continue cutting across, again 1/8 inch below the surface, below your hand. Stop 1/8 inch from the end, turn the meat around and begin a third slice ¼ inch below what is now the top. Cut across 1 inch, unfold the second slice, then continue your cut, 1/8 inch below your firmly held hand. Work your way back and forth across the meat, leaving 1/8 inch hinges at the end of each slice, until the entire piece has been stretched out to a long, 1/8 inch piece of meat.
- Coating and marinating the meat. Coat the meat with marinade and restack it: Spread a scant teaspoon of the paste over the center of a plate, lay the bottom layer of meat over it, spread with a thin cap of marinade, fold over the next layer of meat, spread with marinade, and so on, until the meat has been restacked with a little marinade between all layers. Spread the top and sides with marinade, cover with plastic wrap and refrigerate for several hours or, preferably, overnight.
- Grilling and serving the meat. About 20 minutes before cooking, prepare a charcoal fire, letting the coals burn until they are covered with a gray ash and are medium-hot.
Unfold the marinated meat and cut into smaller, 10 to 12-inch pieces. Lightly oil the grill grates. Gently lay the pieces over the hottest area of the fire. When richly browned, usually just about a minute or two, flip to the other side and cook until medium. Remove to a cutting board and let the meat rest for a few minutes before slicing and serving.
Pass the sliced meat with the Avocado-Tomatillo Salsa and plenty of warm corn tortillas.