In a bowl big enough to hold all the ingredients, mix together the chopped onions and tomatoes, add in the garlic along with the cilantro.
Scoop out the flesh from the avocado, discarding the peel and pit. Add the avocado to the same bowl and crush it with a fork. You can keep it as chunky, or make it as smooth as you like.
Add the lemon juice, peppers, and salt. Taste and adjust the amounts of each of these until they are to your liking.