In a bowl, whisk together soy sauce, sesame oil, brown sugar, minced garlic, grated ginger, rice vinegar, and optional red pepper flakes until well combined. Add the flank steak, ensuring it’s thoroughly coated in the marinade. Cover and refrigerate for at least 30 minutes to allow the flavors to penetrate, ideally up to two hours for extra tenderness.

Prepare your rice according to the package instructions, whether you choose white or brown rice. While the rice cooks, keep an eye on it until it's fluffy and tender, which typically takes about 15–20 minutes. Once done, remove from heat and let it sit covered.

Heat a grill or large skillet over high heat until it's sizzling hot. Remove the steak from the marinade and let any excess drip off. Sear the steak for 4–5 minutes on each side or until it reaches a nice brown crust and is medium rare.

Once cooked, transfer the steak to a cutting board and let it rest for about 5 minutes. Using a sharp knife, thinly slice the steak against the grain.

In each serving bowl, start with a base of cooked rice. Layer the sliced steak on top, followed by your choice of fresh vegetables like thinly sliced cucumber, julienned carrots, and a scoop of kimchi.

Sprinkle sesame seeds over each bowl for a nutty finish and add fresh cilantro if desired. Serve immediately.