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

Place the flank steak in a resealable plastic bag and pour the marinade over it, ensuring it's fully coated. Seal the bag and refrigerate for 1 to 4 hours.

Rinse jasmine rice under cold water until clear, then drain. In a saucepan, combine rice and water, bring to a boil, then reduce to simmer and cover for 15 minutes.

Heat a grill pan over medium-high heat, add vegetable oil. Grill the flank steak for about 4 to 5 minutes per side for medium-rare.

Transfer the cooked steak to a cutting board and let it rest for 5 minutes, covering it loosely with aluminum foil.

Slice the steak against the grain into thin strips and serve over jasmine rice, garnished with cilantro, cucumber, and toasted sesame seeds.