In a medium 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, then pour the prepared marinade over the meat. Seal the bag and refrigerate for at least 1 hour or up to 4 hours.

Rinse 1 cup of jasmine rice under cold water until the water runs clear. In a saucepan, combine rinsed rice with 1.5 cups of water and a pinch of salt. Bring to a boil, then reduce to a simmer, cover, and cook for 15 minutes.

Heat 2 tablespoons of vegetable oil in a grill pan over medium-high heat until it shimmers.

Remove the flank steak from the marinade and cook in the hot grill pan for 4-5 minutes on each side, until medium-rare.

Transfer the steak to a cutting board and cover it loosely with foil. Let it rest for about 5 minutes.

Slice the steak thinly against the grain and serve it over jasmine rice, garnished with fresh cilantro, toasted sesame seeds, and cucumber.