1. Whisk together first 5 ingredients and 4 Tbsp. oil; drizzle lemon juice mixture over steak in a large shallow dish or zip-top plastic freezer bag. Cover or seal, and chill 1 to 12 hours. Remove steak from marinade, discarding marinade. Pat steak dry.
2. Heat remaining 2 Tbsp. oil in a 12-inch cast-iron skillet over medium-high heat. Add steak, and cook, stirring occasionally, 5 to 8 minutes or to desired degree of doneness. Transfer steak to a cutting board, and let stand 10 minutes. Chop steak.
3. Spoon steak into warm tortillas; top with chopped onion and cilantro. Serve with Serrano-Cilantro Salsa and lime wedges.