1) Soak the onion in cold water for 15 mins. Drain and transfer to a large bowl.
2) On a cutting board, lightly sprinkle the garlic with a pinch 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.
3) Using a knife cut the avocados in half, rotating around the seeds. Remove the seeds and discard (See Cook's Note).
4) 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 avocados.
5) Add the coriander, 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.
*Cook's Note: Here's a quick and easy way to remove the seed from an avocado: tap the edge of a cook's knife into the seed and twist--the seed will pop out. Knock the knife on the edge of a counter and the seed will drop off.