Authentic is one of those words which have lost its original meaning due to repetitive erroneous usage.
"Authentic" means "having an origin supported by unquestionable evidence; authenticated; verified"
Now what is "Authentic Indian??"
One may argue that it is the most repeated advertising slogan used by Indian restaurants.
"Authentic Indian food is a food item that is prepared in a way that has originated from India and is Supported by unquestionable evidence"
1. A naan bread when cooked in a clay tandoor is an example of Authentic Indian Food.
2. "Lassi" when made in a clay utensil from pure milk using a wooden blender stick called "madhani".
3. Tandoori roti which is made with pure wheat flour without any egg or plain flour mixed in it.
4. Goat cooked in whole spices on slow flames till the meat starts to fall off the bone.
Authentic then is further divided based on its state of origin. For example an authentic Hyderabadi Dum Biryani is totally different to a Kashmiri Biryani. A north Indian Butter chicken will be different to the Butter chicken that you find in Mumbai. The chilly chicken from Delhi derives its flavours from Soy sauce which is a chineese touch whereas the one found in South India is a very Local flavour.
Also, when you are 7000kms away from the place of origin, then the limitations start taking control of authenticism. Limitation is the quality of clay in the oven, quality of spices, quality of the cooks and chefs and quality of the basic ingredients like rice.
Conclusion being that the term "Authentic" when 7000kms away from its place of origin,particularly with reference to the food, looses its originality and is more of a relative term. The best that you can get is "close to being Authentic ".