Cut the avocados in half, running your knife around the pit from stem to blossom end and back up again. Twist the halves in opposite directions to free the pit, and pull the halves apart. Dislodge the pit, then scoop the avocado flesh into a large bowl. Coarsely mash the avocado with a large fork or potato masher. Rinse the onion under cold water, shake off the excess water, then add it to the avocado along with the serrano, cilantro and lime juice.