How to Make It
In a shallow, heatproof bowl, cover the pasillas with 1 cup boiling water; carefully top with a small plate to keep the pasillas submerged. Let the chiles stand until softened, about 20 minutes. Meanwhile, in a small saucepan, cover the prunes with water and bring to a boil. Cover and let stand until softened, about 10 minutes.
Drain the prunes and transfer to a blender. Transfer the pasillas to a work surface; reserve 1/2 cup of the soaking liquid. Discard the pasilla stems and seeds and coarsely chop the chiles. Add the pasillas and their reserved soaking liquid to the blender and puree.
Light a grill or preheat a grill pan. In a medium saucepan, heat the vegetable oil. Add the onion and garlic and cook over moderately low heat until softened, about 7 minutes. Add the pasilla chile-prune puree along with the sugar, cinnamon and sherry vinegar and bring to a simmer over moderately high heat. Stir in the mezcal and stock and simmer for 1 minute. Season the adobo sauce with salt and pepper.
Season the flank steak with salt and pepper and coat it with 1/4 cup of the adobo sauce. Grill the steak over moderately high heat, turning once, until medium-rare, about 5 minutes per side. Transfer the steak to a carving board and let rest for 5 minutes. Thinly slice the steak across the grain and serve with the remaining pasilla-prune adobo sauce.
Pasilla chiles (also called chiles negros) are long, black, dried chiles. They are available at Latin markets or from gourmetsleuth.com. Mezcal, a Mexican spirit, is available at many liquor stores.