Using a cutting board and sharp knife, finely dice the onion and jalapeno*.
In a mixing bowl, combine the onion, jalapeno, lime juice, and salt. Allow to marinate for 15 minutes.
Slice a 1/4" from the end of each tomato. With the tomato cut-side down, squeeze the seeds and juice from the tomato into a bowl or over the sink. Dice the tomato into small pieces (I cut them into about the size of a fingernail).
Chop the cilantro into small pieces (about the same size as the tomato or smaller).
Add the diced tomatoes and cilantro to the mixing bowl. Stir everything together, and taste for seasoning. If it tastes bland, add more salt to taste.
Let your pico de gallo rest for 15 minutes before serving. Or refrigerate overnight. Serve with tortilla chips or use to garnish your favorite dish.