Heat a cast-iron skillet over high heat. Add onion; cook 5 minutes or until charred, turning once. Remove from pan; chop and place in a bowl.
Heat a small skillet over medium heat. Add 2 tablespoons oil to pan. Add garlic and chiles; cook 2 minutes, stirring occasionally. Add garlic mixture to onion in bowl. Stir in juice and remaining oil. Combine half of onion mixture (including both chiles de árbol) and steak in a large zip-top plastic bag; seal. Refrigerate 1 hour. Reserve remaining onion mixture. Remove steak from refrigerator; let stand at room temperature 1 hour.
Preheat grill to medium-high heat.
Remove steak from marinade; discard marinade. Wipe any remaining onion and garlic off steak. Sprinkle steak with salt. Place steak on grill rack coated with cooking spray; grill over medium-high heat 8 minutes or until desired degree of doneness, turning once. Remove from grill; let stand 15 minutes.
Cut steak across the grain into very thin slices. Add steak and accumulated juices to reserved onion mixture; toss. Place 2 tortillas on each of 6 plates. Top each tortilla with 1 1/2 ounces steak; divide onion mixture among tacos. Top each with 1 tablespoon Toasted Chile Salsa and 1 teaspoon crema.