Heat a grill pan to medium heat and place the avocados and jalapenos in the pan cut-side down. Grill until the avocados and jalapenos start to char, 3 to 5 minutes.
In a medium bowl, douse the garlic and onions with the lemon and lime juices. Season liberally with salt and set aside to allow the flavors to marry.
Scoop the avocado flesh from the skins. Place a metal cooling rack over the bowl. Working with one half at a time, place the avocado cut-side down on the rack and press down, pushing it through the rack. The rack will dice it as its being added to the bowl. Repeat for the remaining halves. Finely chop the jalapenos and add to the guacamole. Mash with a fork until fairly smooth. Adjust the salt, and then serve.