This simple mole relies on ancho chiles, smoky chipotles, roasted peanuts and just a touch of Mexican chocolate.
Heat 2 tablespoons of the oil in 4- to 6-quart heavy pot or Dutch oven over medium. When hot, add onion and garlic. Cook 5 or 6 minutes, stirring frequently, until the onion and garlic have softened and browned. Scrape into a blender jar (setting the pan aside unwashed).
To the blender, add the tomatoes, 1 cup peanuts, bread, ancho powder, chipotles, cinnamon and allspice. Add 2 cups water and blend until as smooth as possible—that takes a couple of minutes with most blenders. A drop of the puree rubbed between fingers shouldn’t feel gritty. (If you are not working with a powerful, high-speed blender, push the mixture through a medium-mesh strainer into a bowl. This will remove anything that didn’t get thoroughly blended.)
Return pan to medium-high heat. Add 2 tablespoons of the remaining oil. When oil is hot enough to make a drop of the puree sizzle fiercely, add it all at once. Stir constantly until reduced by two thirds its original volume and darkened to a thick, rusty-orange paste (about the consistency of tomato paste), 7 to 8 minutes. Use a splatter guard to prevent a mess.
Stir in 2 cups water and the chocolate. Partially cover the pot, turn the heat to low and let it simmer, stirring often, 30 to 60 minutes.
Season the sauce with about 1½ teaspoons salt and ½ teaspoon sugar. Taste and add more salt and sugar as desired. If necessary, adjust the sauce with water to the consistency of a cream soup.
About 1 hour before serving, heat oven to 325 degrees.
Heat remaining 1 tablespoon oil in a 4 to 6-quart heavy pot or Dutch oven. Sprinkle pork generously with salt, then put it fatty side down in the hot oil. Cook, without turning, until nicely browned, about 6 minutes. Flip pork over. Pour the warm mole around (not over) the pork. Cover the pan tightly and bake until the internal temperature of the pork registers 145 degrees on an instant-read thermometer, usually 25 to 30 minutes.
Use tongs to remove the pork loin to a cutting board. Tent with foil and let rest 10 minutes.
Whisk the mole in the pot to insure all the pork juices are thoroughly incorporated. Taste for seasoning, adjusting salt as desired. Thinly slice the pork and arrange on a serving platter. Ladle some of the mole over the pork. Sprinkle with chopped peanuts and cilantro or parsley leaves. Pass remaining mole for guests to add as they wish.