Fire grilling the vegetables softens them slightly while infusing them with a rich, smoky flavor that brings a unique flavor component to the guacamole. This method also works great for salsas, chilis, and stews.
To prevent guacamole from browning, place in a container that does not leave much room for air (pack it tight). Cover the surface of the guacamole with enough onion to completely cover the surface or press plastic wrap over the surface. Seal with lid and refrigerate for up to 5 days.