Soak the onion in cold water for 15 minutes. Drain and transfer to a large bowl.
On a cutting board, lightly sprinkle the garlic with ¼ teaspoon of salt and smash it into a paste with the side of a cook's knife. Transfer the paste to the bowl with the onion.
Using a knife cut the avocados in half, rotating around the seeds. Remove the seeds and discard. Using the knife, carefully make several slashes in the avocado flesh. Using a spoon, scoop the avocado flesh into the bowl with the onion and garlic. Repeat with the remaining avocado.
Add the lime juice, cilantro, tomato, and jalapeno, to taste, to the avocados. Using a rubber spatula or large fork, mix until just combined, keeping the guacamole chunky. Season with salt, to taste. Serve immediately.