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1
Light a grill or preheat a cast-iron grill pan over low heat for about 10 minutes. In a small skillet, toast the cumin seeds over moderately high heat until lightly browned and fragrant, about 30 seconds.
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2
Transfer to a mortar to cool; then, using a pestle, coarsely grind. Add the black pepper and garlic and pound to a paste. (Alternatively, using a chef's knife, chop the cumin seeds as fine as you can and put them in a small bowl along with the black pepper. Mince the garlic and add it to the spices.)
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3
Stir in the olive oil. Rub the cumin oil on the skirt steak and marinate for at least 10 minutes or refrigerate overnight.
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4
Roast the poblano chiles directly on the grill or over a gas flame, turning often, until blistered all over; do not overcook or the flesh will burn.
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5
Transfer the chiles to a paper bag and let steam for about 5 minutes.
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6
Using a small knife, remove the skin, stems, and seeds. Cut the chiles into thin strips.
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7
In a small bowl, mix the chiles and sour cream. Season with salt.
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8
Season the skirt steak with salt and grill over a very hot fire or in the grill pan over high heat for about 2 1/2 minutes per side for medium-rare.
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9
Transfer to a carving board to rest for about 5 minutes. Using a very sharp knife, thinly slice the steak lengthwise against the grain.
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10
Warm the tortillas on the grill for a few seconds until pliable.
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11
Place the sour cream, onion, tomatoes, and avocado in bowls. Serve alongside the steak and tortillas and let each person assemble his own fajita.
