In a food processor, add cilantro stems, lime juice, 1 1/2 Tbs olive oil and 1/2 tsp crushed red pepper. Process until smooth.
Transfer cilantro mixture to a shallow dish. Add flank steak and turn to coat in mixture. Let sit 10 minutes.
Coat the grill with non-stick cooking spray. Remove steak from cilantro mixture and season with salt and black pepper. Place steak on grill and cook for 4 minutes on each side for medium-rare. Transfer to a cutting board and let rest for 5 minutes. Cut against the grain into slices.
Place sliced steak on a serving dish. Top with remaining crushed red pepper, onion, cilantro leaves and garlic. Drizzle with 1 1/2 Tbs olive oil. Serve with lime wedges.