Start out with the best, ripest tomatoes you can find, which unless it's the height of summer, generally means one of the smaller roma, cherry, or plum varieties. I used Campari-brand plum tomatoes for this batch, along with white onion, serrano chilies, lime, and cilantro.
Finely dice your tomatoes by making parallel cuts along all three axes. Start by cutting them in half, then holding them cut-side down, make one or two horizontal cuts. Next, make parallel vertical cuts, and finally rotate 90° and make more vertical cuts to end up with fine dice.
Season the tomatoes with salt, toss them in a fine mesh strainer, and let them drain for about 20 minutes. This will concentrate their flavor and improve texture.
Dice your white onion finely. If it is particularly pungent, it can be rinsed under warm running water.
Finely Dice Chilies
To dice serrano or jalapeño chilies, first remove the end, slice in half, and cut into thin strips, removing the seeds and ribs if you'd like your pico de gallo less spicy.
Rotate and cut
Rotate the strips 90° and cut them into fine, tiny dice.
Combine and season
Toss all the vegetables along with some chopped cilantro and season to taste with lime juice and more salt.
Finished Pico de Gallo
Serve immediately, or refrigerate the pico in a sealed container for up to 3 days. Bear in mind that the onion aroma may get stronger after the first day in the refrigerator.