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. Gently tap pit with a sharp knife, twist and remove the pit. Scoop the avocado flesh into a large bowl. Coarsely mash the avocado with a fork.
Rinse the chopped red onion under cold water (to get rid of some of the excess onion juices/flavor), shake off the excess water, then add it to the avocado along with the chopped chile, cilantro and lime juice.
Mix in two-thirds of the diced mango. Taste and season with salt.
If not serving right away, cover with plastic wrap pressed directly on the surface of the guacamole (to prevent browning) 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 and/or jicama.
Notes
Use plastic gloves when handling the serrano chile or you'll have that spicy burn on your fingers that will be tough to get rid of.