- 2 tablespoons green curry paste
 - 2 tablespoons fish sauce, or more to taste
 - 2 tablespoons rice vinegar, or more to taste
 - 2 tablespoons coconut milk
 - freshly ground black pepper to taste
 - 1 pinch cayenne pepper, or more to taste
 - 1 pinch salt (optional)
 - 2 pounds flap steak, trimmed of fat
 
- Whisk curry paste, fish sauce, rice vinegar, coconut milk, black pepper, cayenne pepper, and salt together in a large shallow glass or ceramic bowl. Add the flap steak and turn to evenly coat. Cover the bowl with plastic wrap and marinate in the refrigerator for 4 to 12 hours.
 - Remove the flap meat from the marinade and shake off excess. Discard remaining marinade.
 - Preheat an outdoor grill for high heat and lightly oil the grate.
 - Cook flap steak on the preheated grill until it starts to firm and is reddish-pink and juicy in the center, 2 to 3 minutes per side. An instant-read thermometer inserted into the center should read 130 degrees F (54 degrees C). Transfer meat to a plate to rest for at least five minutes before slicing against the grain.