Cut the avocado in half around the pit and separate the two halves. Peel the avocado and chop it up into rough chunks and add it to a bowl along with the onion, cilantro, lime juice and salt. You can also use a spoon to scoop out the avocado from the skin, but I recommend peeling it because the most vibrant green color is along the skin, which gets left behind if you use a spoon.

Use a fork to mash up the avocado and stir everything together until there are no huge pieces remaining but the guacamole is still chunky.

Serve immediately. If you need to store the guacamole, spread it flat into a bowl and lay a piece of plastic wrap directly on the guacamole, removing any air bubbles (it's contact with air that oxidizes avocados and makes them turn brown).