Cut the avocados down the middle and twist the halves to expose the pit. Bring down your knife on the pit of the avocado and twist it free using the blade of your knife.
Spoon out the flesh into a medium sized bowl and use a fork to mash to the consistency you like. Add the juice of 1 lime (about a tablespoon), cilantro, and the salt.
Finely mince one shallot and add to the bowl and add cilantro. If you are using red onion I would use about a quarter of a softball sized red onion.
Stir, cover, and chill for about an hour.