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1
In a shallow, heatproof bowl, cover the pasillas with 1 cup boiling water; carefully top with a small plate to keep the pasillas submerged.
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2
Let the chiles stand until softened, about 20 minutes.
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3
Meanwhile, in a small saucepan, cover the prunes with water and bring to a boil.
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4
Cover and let stand until softened, about 10 minutes.
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5
Drain the prunes and transfer to a blender.
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6
Transfer the pasillas to a work surface; reserve 1/2 cup of the soaking liquid.
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7
Discard the pasilla stems and seeds and coarsely chop the chiles.
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8
Add the pasillas and their reserved soaking liquid to the blender and puree.
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9
Light a grill or preheat a grill pan.
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10
In a medium saucepan, heat the vegetable oil.
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11
Add the onion and garlic and cook over moderately low heat until softened, about 7 minutes.
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12
Add the pasilla chile-prune puree along with the sugar, cinnamon and sherry vinegar and bring to a simmer over moderately high heat.
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13
Stir in the mezcal and stock and simmer for 1 minute.
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14
Season the adobo sauce with salt and pepper.
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15
Season the flank steak with salt and pepper and coat it with 1/4 cup of the adobo sauce.
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16
Grill the steak over moderately high heat, turning once, until medium-rare, about 5 minutes per side.
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17
Transfer the steak to a carving board and let rest for 5 minutes.
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18
Thinly slice the steak across the grain and serve with the remaining pasilla-prune adobo sauce.
