1. Brush steak with 1 Tbsp. oil; sprinkle with seasoning.
2. Heat a cast-iron grill pan over medium-high heat; cook steak in pan 7 to 9 minutes on each side or to desired degree of doneness.
3. Process cilantro, next 3 ingredients, and remaining 4 Tbsp. oil in a food processor until smooth. Cut steak diagonally across the grain into thin slices. Serve sauce with steak.