Using a molcajete* (mortar and pestle), mash the jalapeno, onion, and salt together until juices are released from vegetables and onion is soft.
Set pestle aside. Add avocado to mortar and use the side of a metal spoon to break up the avocado. You don't want your guacamole completely mashed and smooth, there should still be some chunks but make to your preference.
Fold in tomatoes and cilantro. Serve with chips or Mexican entree and enjoy.
Notes
* If you don't have a molcajete (mortar and pestle) to make your guacamole, that's totally fine. Use a wooden spoon to mash the onion and jalapeno, and then a metal spoon to mash the avocado.