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.
Mix in 2/3 of the diced mango. Taste and season with salt. If not using immediately, cover with plastic wrap pressed directly on the surface of the guacamole and refrigerate—preferably for no more than a few hours.
When you're ready to serve, scoop the guacamole into a serving bowl and garnish with the remaining diced mango and cilantro sprigs. Serve with tortilla chips, slices of cucumber or jicama.