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

Place the flank steak into a large resealable plastic bag and pour the marinade over it. Seal the bag tightly and refrigerate for 1 to 4 hours.

While the steak marinates, rinse jasmine rice under cold water until clear. Combine washed rice and water in a saucepan. Bring to a boil, then reduce heat and simmer for 15 minutes.

Heat vegetable oil in a skillet or grill pan over medium-high heat. Add the marinated flank steak and sear for 4-5 minutes on each side.

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

Slice the steak against the grain into thin strips and arrange over jasmine rice. Garnish with cilantro, sesame seeds, and cucumber slices.