I may not have tried out every single Indian restaurant in Frankfurt but still it have been a few. And from all these Indian restaurants I've eaten at, the Diya is my favorite. The staff is so friendly and you'll get a nice Indian meal for a good price!
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Serving sizes are good, the taste is delicious (something that's not for certain at other restaurants!), the meat is tender. The interior is not too cliché but still nice enough :) I always go home stuffed and unable to move.
Ask the waiter if they'll make a bathura (fried bread) for you. It's not on the menu, but they know it.