How to Make It
Preheat the oven to 300°. Season the roast with salt and pepper and dust with flour. In a medium, enameled cast-iron pot, heat 2 tablespoons of olive oil. Add the roast; cook over moderate heat, turning, until browned all over, 20 minutes. Transfer to a plate.
Meanwhile, soak the ancho chiles in 1/2 cup boiling water for 20 minutes. In a food processor or blender, puree the chiles with their liquid.
Add the onion and garlic to the pot and cook, stirring, until softened, 5 minutes. Add the chile puree, stock and herb bundle and bring to a boil. Return the meat to the pot, cover and transfer to the oven. Braise for 2 1/2 hours or until tender, turning once halfway through.
Transfer the roast to a plate. Strain the cooking liquid into a bowl; discard the solids. Skim off the fat and return the liquid and meat to the pot. Add the potatoes. Cover and braise in the oven for 30 minutes, until the vegetables are tender.
Transfer the roast to a work surface. Using a slotted spoon, transfer the vegetables to a deep serving platter; cover. Boil the liquid until reduced to 3 1/2 cups. Stir the cornstarch slurry and whisk it into the simmering liquid until thickened.
Remove the strings from the roast and cut into thick slices; arrange on the platter and pour the sauce on top.