1) Add all the ingredients to a medium bowl, and toss to combine. Cover and chill until ready to serve. This salsa can be made four hours ahead.
Notes: In Mexico, pico de gallo is used to describe a wide variety of regional salsas, which are always made with fresh cubed fruit and/or vegetables. The most common version is made with any red tomato, onion and green chiles and is sometimes called "salsa mexicana" or "salsa bandera," because the colors in the salsa represent the Mexican flag.
The serrano chilli is a fresh, small, green chilli. Traditionally used in salsas it is slightly hotter than a jalapeno chile and can be found at most supermarkets, Latin specialty markets or online.