African Eggplant

Actually related to the tomato the African Eggplant is a very acidic fruit. The leaves of the tree are steamed and eaten like spinach and the fruit is used to make a tart relish. Although quite rare in the U.S. you can purchase seeds at Amishland Heirloom Seeds Subject to seasonal availability.

You can use use green tomatoes as an alternative to African Eggplant