In a medium bowl, combine soy sauce, fish sauce, brown sugar, and lime juice. Add minced garlic, black pepper, and red pepper flakes. Whisk until sugar dissolves.

Place flank steak in a resealable bag, pour the marinade over it, seal tightly, and refrigerate for at least 1 hour (4 hours optimal).

Rinse jasmine rice under cold water until clear. Combine rice, water, and salt in a saucepan. Bring to boil, then reduce heat and simmer for 15 minutes. Let sit, covered, for 5 minutes.

Heat grill pan over medium-high heat and add vegetable oil. Grill flank steak for about 4-5 minutes each side until medium-rare.

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

Slice steak against the grain into thin strips. Serve with jasmine rice, topped with cilantro and sesame seeds, with cucumber on the side.