Season the pork on both sides, sprinkle with the vinegar, roll each piece up, and set aside to season for 1 hour. Meanwhile, prepare the filling: Put the chorizo into a skillet and cook over low heat until all the fat has rendered out. Discard all but 2 tablespoons of the fat. Stir in the potatoes and onion and cook over low heat for about 2 minutes. Add salt as necessary. Set aside to cool and then mix with the beaten egg. You should have about 2-1/2 cups (625 ml).

Divide the filling into half and spread each of the pieces of meat with it. Roll each piece of meat and tie firmly-but not too tightly or the stuffing will extrude.

If not using a pressure cooker, place the meat in a heavy pan, closely seal the lid, and cook over low heat, shaking the pot from time to time, for about 45 minutes. The meat should be just tender.

Meanwhile, make the sauce: Put the chiles and 1 cup (250 ml) of the water plus the vinegar into a small pan, bring to a simmer, and cook for 5 minutes. Set aside to soak for 5 more minutes. Put 1/2 cup (125 ml) of the water into a blender, add the garlic, spices, oregano, and sesame seeds, and blend as smoothly as possible. Add the remaining 1/2 cup (125 ml) water, a few of the chiles, and the bread and blend until fairly smooth. Add the remaining chiles and again blend as smooth as possible. The sauce should be thick-don't dilute further unless absolutely necessary to release the blender blade.

Remove the meat from the broth, reserving the broth for the sauce. Heat the lard in a heavy casserole, add the meat rolls, and brown them lightly, rolling them over from time to time. Add the sauce and reduce over fairly high heat, scraping the bottom of the pan to prevent sticking, then add the reserved broth and continue cooking over fairly high heat, adding salt as necessary until the meat has absorbed the flavors of the sauce-about 15 minutes. Remove the string, slice the meat, and serve with plenty of the sauce.