We went to Malik's Tandoor Fine Asian Cusine this week (10841 Bustleton, very close to the BJ's). Although the name seemed sketchy to me, we had a recommendation from a friend. It was among the most authentic Indian restaurants I have been to (possibly the most authentic).
We got butter chicken (an actual half chicken, like what you'd get in Delhi) with just the right type and amount of spices. They were very generous with the nan, and for the first time in the USA, I drank masala chai that was almost exactly they way they make it in Delhi. Malik's is attached to a Indian-Pakistani grocery store, but we finished dinner too late to check it out.