Cut the avocado in half, remove the pit, and scoop out the flesh.
Place the avocado flesh in a bowl and mash it with a fork.
Finely chop the onion, tomato, cilantro, and jalapeño.
Add the chopped vegetables to the avocado and mix together.
Squeeze lime juice in and season with salt to taste.
Mix until smooth and add more cilantro if desired.