Minera (with another branch in Herdon, although I haven't been) offers a decent lunch buffet. The selection is good and always includes tandori chicken, a few curries, vegitarian dishes, 2 types of bread, 3 or 4 chutnies (the ginger is excellent!), one chinese dish (usually lo mein), fruit, salad, Indian desert (rice pudding), and more. I almost always go with natives of various parts of India, Pakistan, and Bangledesh and they tell me that the food is authentic enough.
Don't expect anything fancy. It's a huge and plain dining room with two buffets (one hot, one smaller cold one). Make sure to pick up you own forks, knives, and spoons near the cold buffet.
Lunch here will run you about $10 or so, with tax and tip. Not bad considering that you can get your fill.
For dinner, they offer a regular menu. I've never tried it, but it has everything you'd expect.