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.
Heat a grill on high. Add steak to grill, and cook for approximately 5 minutes per side for medium rare. Remove and tent with foil; let rest for ten minutes (definitely at least 5).
Meanwhile, heat a pan over med high/high heat. Add a small amount of vegetable oil to the pan and add peppers and onions to pan. Cook until vegetables begin to soften. Season with 1/2 tsp garlic salt and pepper. Keep warm. Heat tortillas on grill, 1 min per side with grill on low.
Slice skirt steak against the grain and serve with peppers and onions, yogurt, tomato, cheese and cilantro.