Combine white onion, jalapeño, cilantro, lime and salt in the molcajete. If you don't have a molcajete, use a mortar and pestle or just mash in a bowl.
Pound for 5 minutes or until it becomes a thick paste.
Split the avocado in half, discard the pit, and scoop out the flesh.
In a separate bowl add the avocado, mash and keep chunky.
Combine the paste and avocado, mix well, taste for seasoning, and enjoy!