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1
Fire up a grill or grill pan over medium-high heat. You should be able to hover your hand a few inches above the grill surface for 5 seconds or so, but not more, before you'll need to pull away. That's hot enough.
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2
Toast the chiles on the fire for about 1 minute, turning once. A little char is good; burned is not.
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3
Pull the stem end off of the chiles, shake out most of the seeds, and grind the chiles in a food processor.
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4
Add the sugar, salt, and garlic to the ground chiles in the processor bowl. Pulse until well combined.
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5
Massage the steak deeply with the ancho rub. Flap meat is quite cavernous, so use your thumbs and fingers to press the rub into all the cracks and crevasses.
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6
Spray a clean grill rag with cooking spray. Rub into the grill grates until they glisten.
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7
Place the lime on the grill, cut sides down, and char for about 2 to 3 minutes before removing from the grill. Set aside in a small bowl.
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8
Grill the steak, about 4 minutes on each side for medium-rare. Don't tinker with it. Allow for some uneven charring.
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9
Transfer the steak to a cutting board to rest for 5 minutes.
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10
Cut on a sharp bias, with your knife nearly parallel to the board, in very thin slices.
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11
Squeeze the grilled lime halves over the sliced beef. Serve.
