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Peel and chop the red onion, garlic, jalapeño and cilantro. Put in more or less garlic and jalapeño according to your taste. Place in a mortar with the salt. Grind everything together with a pestle into a coarse paste. Transfer the paste to a bowl.
Cut the avocados in half and remove the seeds.
Score the flesh of each while still in the skin. Scoop the flesh out with a spoon and add to the bowl with the paste. Squeeze the lime over the mixture and toss.
If you like your guac chunky, use a potato masher or fork to mash the avocados. Stir to combine evenly with the paste. If you like it smooth, put everything into a food processor and process til no lumps remain.
Serve with potato chips, tortilla chips, celery sticks, carrot chips, or other dipping vehicle. You can also use guacamole as a spread or filling for pita sandwiches or a topping on a salad. Tomatoes are an excellent accompaniment to this dip.
It's best consumed fresh, but if you have to refrigerate it, press plastic wrap directly on the surface of the guacamole, squeezing out as much air as possible. Avocados turn an ugly brown when exposed to the slightest amount of air, so seal it as tightly as possible.