First, prepare the tomatoes: In a saucepan, bring water to a boil. Make a small incision on the bottom of the 2 cherry tomatoes and dip them into the boiling water for a few seconds. This helps the skin peel off easily. Once peeled, remove the seeds and cut the tomato flesh into small cubes.
Wash and finely chop the 1 onion. Cut the 1 avocado in half, remove the pit, and scoop out the pulp with a spoon. Transfer the pulp to a plate and mash it with a fork until it's mostly smooth but still has some texture. Stir in the juice of half a lemon and the chopped onion.
Finally, add the chopped tomatoes and 1 tablespoon parsley to the avocado mixture. Mix everything until just combined. Taste the guacamole and adjust the seasoning with salt and bell pepper as needed.