Press each avocado near the stem end to confirm it yields slightly under firm pressure. Slice in half lengthwise, twist apart, and remove the pit with a spoon.
Scoop the flesh into a medium bowl. Add lime juice and salt immediately. Mash with a fork by pressing and folding until you have roughly 60% smooth and 40% visible chunks.
Fold in the red onion, jalapeño, and cilantro using 8 to 10 gentle turns of a spoon.
Fold in the diced tomato. Taste and adjust: more lime juice if flat, more salt if needed, more jalapeño for heat. Add cumin if using.
Rest uncovered for 5 minutes before serving.