Mince the garlic and set it aside for 10 minutes to allow its healthful compound allicin to develop.
Wearing gloves to keep the powerful oils off your skin, slice the jalapeño in half lengthwise and use a small spoon to scrape out the ribs and seeds, and then cut it into a small dice.
On a large cutting board, chop the onion into a large dice and keep chopping as you add in the garlic, jalapeños, and cilantro to meld them together.
Halve the avocados and use a spoon to scoop all the flesh into a large bowl.
Coarsely mash the avocado with a fork or potato masher and then fold in the chopped vegetables.
Squeeze the juice of one half of the lime over the guac and fold in the cumin, a few twists of your black peppercorn grinder, and a sprinkle of salt.
Taste and adjust the seasonings, adding more lime juice, black pepper, and/or salt as desired.
Just before serving, top with a sprinkle of black lime.