Prepare Marinade: In a mixing bowl, combine soy sauce, fish sauce, brown sugar, lime juice, minced garlic, black pepper, and red pepper flakes. Whisk until the brown sugar dissolves.

Marinate Steak: Place flank steak in a resealable bag, pour in marinade, seal and massage it gently. Refrigerate for at least 1 hour, up to 4 hours.

Cook Rice: Rinse jasmine rice under cold water. In a pot, combine rice with 1.5 cups water and a pinch of salt. Bring to a boil, reduce heat, cover, and simmer for 15 minutes. Let rest for 5 minutes.

Cook Beef: Heat vegetable oil in a grill pan over medium-high heat. Cook flank steak for 4-5 minutes on each side until medium-rare.

Rest Steak: Transfer steak to a cutting board, cover loosely with foil and rest for 5 minutes.

Assemble Dish: Slice the rested steak against the grain. Serve over jasmine rice and garnish with fresh cilantro, toasted sesame seeds, and cucumber slices.