Scoop the flesh from the avocados using a spoon and place in a mixing bowl. Add the onions, jalapeño, and cumin (if using) to the avocado and using a fork, mash the avocado together with the onion and jalapeño, leaving some chunks of avocado for texture. Mix in the cilantro, lime juice, salt and pepper. Refrigerate for 1 hour before serving.
Notes
Allowing this guacamole to sit for an hour before serving is crucial! It allows all the flavors to meld together.I don't recommend making this more than a couple of hours in advance before serving.