Begin by marinating the flank steak. In a bowl, combine soy sauce, fish sauce, lime juice, brown sugar, minced garlic, and red pepper flakes. Mix well until the sugar dissolves.

Place the flank steak in a resealable plastic bag or a shallow dish and pour the marinade over it. Seal the bag or cover the dish and refrigerate for at least 1 hour, or up to 4 hours for more flavor.

Preheat your grill or a grill pan over medium-high heat. Lightly oil the grill grates with vegetable oil.

Remove the steak from the marinade and let any excess marinade drip off. Discard the marinade.

Grill the steak for about 4-5 minutes on each side for medium-rare, or until it reaches your desired doneness.

Once cooked, remove the steak from the grill and let it rest for 5 minutes before slicing.

Slice the steak against the grain into thin strips.

Serve garnished with fresh cilantro and lime wedges on the side.