Combine the onion, cilantro, lime juice and salt in a medium size bowl. Allow to sit for a few minutes.
Meanwhile, slice the jalapeno in half. Scrape out the seeds with a spoon. If you like it spicy, you can leave the seeds in. Then dice the jalapeno finely. Add 1-3 teaspoons depending on the level of heat desired. Toss with the onion mixture and let sit another minute.
Next add the tomatoes to the bowl, draining the juices before adding. Stir the tomatoes in gently and then salt to taste. Add more jalapeno or cilantro if desired.