Chop the cilantro leaves and tender stems. Discarded the tougher stems.
Seed and devein the jalapeño. Dice it finely.
Thinly slice the green onion.
Peel the garlic cloves and the avocados and remove the avocado pit.
Mash the avocados into a chunky paste.
Stir in the jalapeño, cilantro, onion, and tomato if using. Grate in the garlic cloves and squeeze in the juice from a lime. Stir just until combined.