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

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

Rinse jasmine rice under cold water; combine with water and salt in a saucepan, boil, cover, reduce heat, and simmer for 15 minutes.

Heat vegetable oil in a grill pan over medium-high heat, add the marinated flank steak, and cook for 4-5 minutes on each side for medium-rare.

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

Slice the rested steak against the grain, arrange on jasmine rice, garnish with cilantro and sesame seeds, serve with cucumber slices.