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.
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.)
Stir in the olive oil. Rub the cumin oil on the skirt steak and marinate for at least 10 minutes or refrigerate overnight.
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.
Transfer the chiles to a paper bag and let steam for about 5 minutes.
Using a small knife, remove the skin, stems, and seeds. Cut the chiles into thin strips.
In a small bowl, mix the chiles and sour cream. Season with salt.
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.
Transfer to a carving board to rest for about 5 minutes. Using a very sharp knife, thinly slice the steak lengthwise against the grain.
Warm the tortillas on the grill for a few seconds until pliable.
Place the sour cream, onion, tomatoes, and avocado in bowls. Serve alongside the steak and tortillas and let each person assemble his own fajita.