Depending on how many tomatoes, choose two matching size plastic lids.
Put the tomatoes on the top side of one lid. The rim will prevent the tomatoes from rolling off. Make sure to use enough tomatoes to cover the whole lid.
Flip the second lid over onto the tomatoes so that the rim is upside-down, or facing the bottom rim.
Grab your knife and...
Holding the top lid firmly, start cutting between the two lids.
It is best to use a sawing motion while cutting.
Voila! You can cut a lot of tomatoes quickly using this method.