My friend owns a restaurant in 삼청동 SamCheong-dong called "The Restaurant". Very clever name I know, but the food is great, and it's also attached to an art gallery as well. The surrounding area is also very nice with lots of other galleries, tea shops, etc...
http://www.the-restaurant.co.kr/kor/contact.php
Another place that's nice to go to on Christmas Eve is the "Sky Revolving Restaurant" in Seoul Tower on Nam San (south mountain) but I can almost guarantee that this place will be packed. I don't recall if they do reservations, but it would be worth the shot. The restaurant revolves giving you a nice 360 degree view of the city, which is all the better at night.
Both places are great, but neither is all that cheap. Hope this helps
