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 is fully dissolved.

Place the flank steak in a resealable bag or shallow dish, then pour the prepared marinade over it. Seal and refrigerate for at least 1 hour.

Rinse the jasmine rice under cold water until the water runs clear. In a medium pot, combine the rinsed rice with water, using a 1:1.5 rice-to-water ratio. Bring to a boil, then reduce to low, cover, and simmer for about 15 minutes.

Heat a grill pan over medium-high heat, add a splash of vegetable oil. Grill the marinated steak for 4-5 minutes on each side until medium-rare. Let it rest for 5 minutes.

Thinly slice the steak against the grain. Garnish with cilantro and serve with cucumber slices on the side.