In a medium bowl, whisk together soy sauce, fish sauce, brown sugar, lime juice, minced garlic, black pepper, and red pepper flakes.

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

Rinse jasmine rice under cold water until clear, combine with water in a saucepan, bring to a boil, cover, and simmer for 15 minutes.

Heat vegetable oil in a skillet over medium-high heat. Add the marinated steak, searing for 4-5 minutes on each side.

Transfer the steak to a cutting board and let it rest for 5 minutes.

Slice the flank steak against the grain into thin strips.

Serve jasmine rice on plates, top with sliced beef, sprinkle with cilantro and sesame seeds, and add cucumber slices.