Place cilantro stems, juice, 1 1/2 tablespoons oil, and 1/2 teaspoon crushed red pepper in a mini food processor; process until smooth. Place cilantro mixture in a shallow dish; add steak, turning to coat. Let stand 10 minutes.
Coat grill grate with cooking spray. Remove steak from marinade; discard marinade. Sprinkle steak with salt. Add steak to grill; cook 4 minutes on each side for medium-rare or until desired degree of doneness. Place steak on a cutting board. Let stand 5 minutes. Cut across the grain into thin slices.
Place steak on a platter; sprinkle with remaining 1/4 teaspoon crushed red pepper, onion, cilantro leaves, and garlic. Drizzle with remaining 1 1/2 tablespoons oil. Serve steak with lime wedges.