Cut around each avocado, from stem to blossom end and back again, then twist the two halves apart. Dislodge the pit and scoop the avocado flesh into a bowl. Using an old fashioned potato masher or a large fork or spoon, mash the avocados into a coarse puree. Scoop the onion into a small strainer and rinse under cold water. Shake off the excess water and mix into the avocado, along with the chiles, tomatoes and cilantro. Taste and season with salt and lime juice--the guacamole usually takes about 1 teaspoon of salt; lime juice is a matter of personal preference.
Cover with plastic wrap directly on the surface of the guacamole and refrigerate until you're ready to serve (for best results, this needs to be within a couple of hours). Scoop the guacamole into a serving dish, sprinkle with a little chopped cilantro and queso fresco (if you're using it) and you're ready to serve.
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