Scoop the avocado flesh into a medium bowl and mash with a fork until mostly smooth but still slightly chunky.

Add the diced red onion, lime juice, cilantro, jalapeño, salt, and cumin to the mashed avocado.

Gently fold in the diced tomato, being careful not to over-mix.

Taste and adjust seasoning with additional salt and lime juice if needed.

Serve immediately or press plastic wrap directly onto the surface to prevent browning and refrigerate for up to 2 hours.