Combine the oil, soy sauce, scallions, garlic, lime juice, red pepper, cumin, and sugar in a blender carafe and puree. Place skirt steak in a large heavy duty, zip-top bag and pour in marinade. Seal bag, removing as much air as possible. Allow steak to marinate for at least 1 hour in refrigerator.
Fire up one chimney starter or natural lump charcoal. When the coals are white and ashy, distribute them evenly in a charcoal grill.
Remove the steak from the bag and pat dry with paper towels. Using a blow dryer, blow charcoal clean of ash. Immediately lay the steaks directly onto the hot coals and cook 1 minute per side. When finished cooking, wrap meat in a double layer of aluminum foil and allow to rest for 15 minutes.
Remove meat from foil, reserving foil and juices. Slice thinly across the grain and return to foil pouch and toss with juice. Serve with grilled peppers and onions, if desired.