"A piece of tomato"
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.