Open bottom vent completely. Light large chimney starter mounded with charcoal briquettes (7 quarts). When top coals are partially covered with ash, pour evenly over half of grill. Set cooking grate in place, cover, and open lid vent completely. Heat grill until hot, about 5 minutes.
Brush grill grate clean, then brush grill grate with oil. Arrange 20 scallions on cooler side of grill with white ends towards hotter end of grill. Arrange 2 jalapeños on hot side of grill. Cook, covered, flipping scallions and rotating jalapeños every 2 minutes, until scallions are well-charred, 7–10 minutes, and jalapeños are blistered and charred in spots, 10–12 minutes.
Transfer scallions and jalapeños to a large bowl and cover tightly with plastic wrap; set aside.
Arrange steak on hotter side of grill and cook, covered, flipping steak, every 3 minutes, until meat is well browned and an instant-read thermometer inserted in the centers registers about 135ºF (57ºC) for medium-rare, 7–10 minutes total. Transfer steak to clean cutting board; tent with foil.
Arrange 6 tortillas on hotter side of grill and cook until lightly charred, 45–60 seconds per side. Wrap warmed tortillas tightly in foil or a clean kitchen towel. Repeat with remaining 6 tortillas.
Without peeling, stem and seed jalapeños; reserve seeds. Finely chop jalapeños and transfer to bowl. Chop scallions and transfer to bowl with jalapeños. Stir in remaining 2 tablespoons oil, 1 teaspoon adobo sauce, ½ teaspoon kosher salt, and 2 tablespoons lime juice. Season to taste with salt, extra lime juice, and reserved jalapeño seeds for spice.
Thinly slice steak across the grain; transfer to a serving platter. Serve steak in tortillas, with charred scallion salsa, lime wedges, cilantro, and crema separately.