"A piece of tomato"
I wasn't sure what to write, so I tried "Kawałek pomidora" (correct). I was thinking of Kawałek because of Wedel and the like. Is that a good translation or is there something more natural?
That would kinda be "one whole tomato", but it would sound really weird anyway.
Although English can use the word "piece" in this context (at least so it seems from Wiktionary), imagine a table from IKEA that you have to assemble yourself. In the instruction you will have a picture of a bolt/screw/whatever and it will say that there are "8 sztuk" of such element. So basically 8 identical elements.