Preheat broiler. Trim fat from steak. In a small bowl combine salt, black pepper, and cayenne pepper; sprinkle evenly over steak. Place steak on the unheated rack of a broiler pan. Broil 3 to 4 inches from the heat for 8 to 12 minutes or until desired doneness, turning once. Cover with foil; let steak stand for 5 minutes.
Meanwhile, in a large skillet heat oil over medium heat. Add onions; cover and cook about 12 minutes or until very tender and brown, stirring occasionally. Turn heat to medium-low if onions start to get too brown.
Thinly slice steak across the grain. Divide steak, onions, and mango among tortillas. Sprinkle each with cilantro and serve with lime wedges for squeezing. Serve immediately.