Lay the flank steak flat and seal in a shallow lidded container or sheet of plastic wrap. Freeze for about 30 minutes to make slicing easier.

To make the marinade: Thinly slice the garlic and place in a shallow mixing bowl. Add the mirin, soy sauce, sesame oil, brown sugar, salt, and pepper and stir until sugar dissolves.

Slice the steak on a diagonal across the grain into 1/4-inch-thick slices. Add steak to the marinade and mix until each slice is well coated. Cover the bowl and refrigerate for at least 8 hours and up to 24 hours, turning the meat occasionally in marinade.

Heat a grill pan or large frying pan over medium-high heat until hot. Brush oil onto the grates. Grill the steak flat in a single layer in 2-3 batches, turning as needed, until steak is lightly charred and cooked through, 1-3 minutes total. If using a frying pan, add oil as needed to prevent sticking. Transfer the meat to a serving platter as it is finished.

Slice the green onions on a diagonal and scatter over the bulgogi. Serve beef with rice, lettuce, and kimchi if desired.