Scoop out avocado from the peel, put in a mixing bowl.
Using a fork, mash the avocado.
Add the chopped onion, garlic, cilantro, lime or lemon, salt.
Cover with plastic wrap directly or place in an airtight container to prevent oxidation from the air reaching it. Refrigerate until ready.
Just before serving, add the chopped tomato to the guacamole and mix.