Heat charcoal, preferably natural chunk, until grey ash appears. In a blender, put in oil, soy sauce, scallions, garlic, lime juice, red pepper, cumin, and sugar and puree. In a large heavy duty, zip top bag, put pieces of skirt steak and pour in marinade. Seal bag, removing as much air as possible. Allow steak to marinate for 1 hour in refrigerator.
Remove steak from bag and pat dry with paper towels. Using a blow dryer, blow charcoal clean of ash. Once clean of ash lay steaks directly onto hot coals for 1 minute per side. When finished cooking, place meat in double thickness of aluminum foil, wrap, and allow to sit for 15 minutes.
Remove meat from foil, reserving foil and juices. Slice thinly across the grain of the meat. Return to foil pouch and toss with juice. Serve with grilled peppers and onions, if desired.