Although this restaurant can be difficult to find, the food is divine!
I am personally fond of the Makhani, a mild, creamy sauce with an accompanying meat. You can get it with naan for 11,000 won (9,000 won during lunch times 11:00-3:00). They have curry, tandoori, vindaloo, and more at reasonable prices. The great thing about this place is you get to pick the number of spiciness 1-10. The owners say they usually stick to a 7, and refuse to go up to 10, but if there are any brave souls in your group they might want to risk it! I usually stick with a 3 because I'm a wuss, which means I get really mild, delicious, full-flavored food. Lucky me!
Take Line 7 to Sang Dong station and take exit 3 or 4. You will have exited onto a street corner. Turn and look at the Lotteria. The restaurant is on the second floor, directly across the street from KEB. It has lights in the shape of the Taj Mahal in their window, so it may be easier to find at night.