Forget Shin-Okubo's Koreatown. When I asked Korean friends and acquaintances where to go to find authentic Korean food in Tokyo, several pointed me in the opposite direction, to Akasaka. Specifically, I was told to try the soup at Akasaka Ichiryu Bekkan (2-13-17 Akasaka, Minato-ku, Tokyo; 03-3582-7008).
There's really only one thing on the menu at Ichiryu Bekkan: seolleongtang, a beef-bone soup largely associated with Seoul. The bones are cooked all day so that the marrow leaches out into the broth, giving the soup a milky white color and a rich, nearly creamy texture. At Ichiryu it is served with a few chunks of tender beef and a generous handful of scallions. You salt it yourself, to taste, at the table.
The menu may be limited, but that's not taking into account the banchan, the side dishes of kimchi, marinated vegetables and so on that are a big part of any Korean meal. At Ichiryu Bekkan, I counted 11 dishes.