Place the avocado flesh and the juice of the lime in a large bowl; toss to coat, then drain, reserving the juice, and set aside.
Add the cumin to the avocado and mash together with a potato masher. Stir in salt to taste.
Remove seeds from the tomato and chop into small pieces, along with the onion and the cilantro; mince the garlic. Fold all into the mashed avocado, along with 1 tablespoon of the reserved lime juice.