Chop onion into small dice (about 1/8") and place in a large mixing bowl.

Slice avocados in half and remove the pits, saving the pits for later.

Hold one of the avocado halves in your non-knife hand, cupping the skin against your palm. With a paring or chef's knife, score each avocado half by making four or five evenly spaced cuts through the flesh down to the skin but not piercing the avocado skin (or yours). Rotate the avocado 90 degrees in your hand and make four or five more slices so that the avocado is scored in a crosshatch pattern. Repeat for the other halves.

Using a large soup spoon, scoop the flesh of each half into the mixing bowl containing the onion.

Using a fork or pastry cutter, mash the avocados together with the onion just until there are no large chunks of avocado left.

Chop tomatoes into 1/4 dice and add to the mixing bowl.

Add half of the sea salt and half of the lime juice and mix thoroughly. Add additional salt and/or lime juice to taste.

When the guacamole is to your liking, place in an airtight container with the pits and refrigerate until it's time to leave for the party. You can make this a day ahead and just give a quick stir before the party to restore the bright green color.