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

Place the flank steak in a resealable plastic bag, pour the marinade over it, seal the bag, and refrigerate for 1 to 4 hours.

Rinse jasmine rice under cold water until clear. Combine rinsed rice with water in a saucepan, bring to a boil, reduce heat and simmer for about 15 minutes.

Heat vegetable oil in a skillet over medium-high heat. Remove steak from marinade and sear in the skillet for 4 to 5 minutes on each side.

Let cooked flank steak rest for 5 minutes on a cutting board before slicing against the grain into thin strips.

To serve, place jasmine rice on each plate, top with steak slices, and garnish with cilantro and cucumber. Optionally drizzle with leftover marinade.