This potato-based curry has a delicious flavorful taste already, but if you really want it to have an extra tangy taste, you can add a bit of tomato purée or tomato paste into the curry mixture and allow it to cook for a few additional minutes.  Remember that adding the tomatoes will make the gravy slightly runny, so you might need to let it simmer for a while to let all that extra liquid evaporate, especially if you want your curry to be on the drier side.