Heat a large cast-iron skillet over medium-high heat. Add the cumin seed and cook, stirring occasionally, until toasted, about 1 minute. Transfer to a small bowl. Add the chilies and garlic to the skillet. Cook over medium high heat, turning occasionally, until both are soft and blotchy black in spots, about 5 minutes. Remove from the skillet and cool. Add the chilies, garlic, toasted cumin seeds, oil, brown sugar and lime juice, 1 1/2 teaspoon salt and a few grinds of pepper to a blender and process until very smooth. Transfer the marinade to a large re-sealable plastic bag and add the steak. Shake well to coat, refrigerate for 1 to 4 hours.
Prepare a grill for high heat.
Remove the steaks from the marinade and shake off any excess. Brush the grill grates lightly with oil, lay the steak on the grill and cook until very well charred and grill marks appear, about 5 minutes. Turn the steaks and continue to cook until nicely charred, about 1 to 3 minutes more depending on thickness. Transfer the steaks to a cutting board and let rest for 10 minutes. While the steak rests, brush the scallions lightly with oil and grill, turning once until wilted and charred, about 2 minutes. Coarsely chop.
Warm the tortillas on the grill grates. Thinly slice the steak against the grain on a bias. Double the tortillas then top with the steak, grilled scallions, guacamole and pico de gallo. Serve with lime wedges.
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