Cut the avocado in half, remove the pit, and scoop out the flesh with a spoon.
Place the avocado in a bowl and mash it with a fork.
Finely chop onion, tomato, cilantro, and jalapeno.
Add the chopped onion, tomato, cilantro, and jalapeno to the mashed avocado.
Add lime juice, salt, and pepper, then mix well.