In a medium bowl, combine the soy sauce, sesame oil, brown sugar, minced garlic, grated ginger, rice vinegar, and red pepper flakes (if using). Whisk until the sugar is dissolved.

Place the flank steak in a large resealable plastic bag or shallow dish. Pour the marinade over the steak, ensuring it’s well coated. Seal the bag or cover the dish with plastic wrap.

Refrigerate for at least 30 minutes, or up to 4 hours for maximum flavor.

Rinse uncooked rice under cold water until the water runs clear to remove excess starch.

Cook the rice according to package instructions (1 cup of rice with 2 cups of water). Bring to a boil, then reduce heat to low, cover, and simmer (45 minutes for brown rice, 15-20 minutes for white rice).

Fluff the cooked rice with a fork and keep warm.

Heat a grill pan or cast-iron skillet over medium-high heat (or preheat a grill to high).

Remove the steak from the marinade, allowing excess marinade to drip off. Discard the marinade.

Add the steak to the hot pan and cook for 4-5 minutes on one side without moving it.

Flip the steak and cook for another 4-5 minutes for medium-rare, or longer for desired doneness (130°F for medium-rare, 140°F for medium, 150°F for medium-well).

Remove the steak from heat and let it rest for 5-10 minutes.

Slice the rested steak against the grain into thin strips for tenderness.

In a bowl, start with a generous scoop of cooked rice as the base.

Top with sliced steak, kimchi, cucumber, carrots, avocado, and green onions.

Garnish with sesame seeds and fresh cilantro if desired.