1/2cupcold dark beerI recommend using a German beer, since all of those are vegan.
Pour about a half inch of oil into a skillet and let it heat while mixing the batter together in a mixing bowl. Combine flour, cornstarch, and baking powder, and then make a well in the center of the flour. Pour in the beer and water and mix together until thoroughly combined.
Dip each slice of tomato into the batter. The batter on the tomatoes is slippery, and so it will require some pressing and manipulating of the batter to get it to stick. For this reason, it's important that the oil is hot enough when you are ready to fry; otherwise, you could end up with the batter simply falling off of the slices. Test the oil by dropping in a dollop of the batter. If bubbles rapidly form around it then the oil is hot enough.
Fry the slices until they are golden, about three minutes on each side. Place the tomato slices on paper towels or a slotted baking sheet to drain and lightly shake salt over them while they are still hot.
Adapted from Beer Battered Tofu recipe in Vegan Brunch