In a mixing bowl, combine soy sauce, sesame oil, brown sugar, minced garlic, grated ginger, rice vinegar, and optional red pepper flakes. Add the flank steak, ensuring it is fully coated in the marinade. Cover the bowl and refrigerate for at least 30 minutes.

While the steak marinates, rinse and cook your choice of rice according to package instructions, typically around 15-20 minutes for white rice or 40-45 minutes for brown rice.

Preheat your grill or a heavy skillet over medium-high heat for about 5 minutes. Remove the marinated steak from the fridge and shake off excess marinade. Grill or sear the steak for approximately 4-5 minutes on each side.

Once the flank steak reaches your desired doneness, transfer it to a cutting board and let it rest for 5 minutes. After resting, thinly slice the steak against the grain.

In serving bowls, start by layering cooked rice as your base. Next, artfully arrange the sliced steak over the rice. Top each bowl with a generous helping of kimchi, fresh cucumber slices, julienned carrots, and creamy avocado. Sprinkle with chopped green onions, sesame seeds, and fresh cilantro.