Place skirt steak on a rimmed baking sheet; rub olive oil, 3/4 teaspoon salt, and pepper on both sides. Cover with plastic wrap; refrigerate 1 hour. Preheat a charcoal grill to medium. Wrap onions in foil. When charcoal is covered with gray ash, nestle onions directly in the coals; grill 25 minutes or until charred outside and soft within. Cut onions into quarters; separate onion layers with a fork. Place on a platter. Drizzle with lime juice and remaining salt; keep warm. Add coals to fire; preheat to high. Place steak on grill rack coated with cooking spray; grill 5 minutes on each side or until desired degree of doneness. Place steak on a cutting board; let stand 10 minutes. Cut steak against the grain into 1/4-inch strips. Place tortillas on grill rack coated with cooking spray; grill 30 seconds or until heated. Place about 1 1/2 ounces meat on each tortilla; top with about 2 tablespoons onion mixture.