Place the cilantro, lime juice, oil, garlic, green onions, Serrano chile, and sea salt together in a bowl. Blend with an immersion blender until smooth. Place a flank steak into a large zip lock bag and pour half of the marinade into the bag. Squish the bag so the marinade is completely covering the flank steak. Place into the refrigerator and let marinate for 3-4 hours. Reserve the other half of the marinade as a dipping sauce.