Any leftover marinated steak is perfect for serving over a salad or in sandwiches.
In a mini food processor, combine the basil, cilantro, garlic, sambal oelek and fish sauce and pulse until finely chopped. Add the lemon and lime zests and oil and pulse until fairly smooth.
Rub the marinade on the steaks and let stand at room temperature for at least 30 minutes or refrigerate for up to 24 hours.
Light a grill. Grill the steaks over moderately high heat, turning occasionally, until medium-rare, about 6 minutes for skirt steak and 12 minutes for the flank. Let the steaks rest for 5 minutes, then thinly slice across the grain and serve.
Variation This marinade is great for shrimp and chicken.