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

Place flank steak in a resealable plastic bag and pour the marinade over it. Seal and refrigerate for at least 1 hour, ideally up to 4 hours.

Rinse 1 cup of jasmine rice under cold water until clear. Combine rinsed rice with 1 ¾ cups of water and a pinch of salt in a saucepan. Bring to a boil, cover, and reduce to low for about 15 minutes.

Preheat grill pan over medium-high heat and add vegetable oil. Remove flank steak from marinade and place in the hot grill pan. Cook each side for 4-5 minutes.

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

Slice flank steak against the grain into thin strips. Arrange over jasmine rice, garnishing with cilantro and sesame seeds. Serve with cucumber slices.