Put onion, cilantro and peppers in a food processor and pulse until very finely chopped. You can also choose to do this by hand, but it's quite tedious.
Transfer the processed veggies to a bowl and stir in the tomatoes. Squeeze lime juice over the top, and stir. Season with about ½ teaspoon kosher salt and ¼ teaspoon pepper.
Cover and refrigerate for several hours, allowing the flavors to blend before serving. Best if served the same day as prepared.