Using a chopper or sharp knife dice up the onion and soak it in a bowl of cold water to reduce the strong oniony taste.
After the onion has soaked for about 20 minutes, strain out the water using a mesh strainer and set aside.
Using a tomato corer or knife, remove the core from the tomato. Cut the tomato in half and dice it using a chopper or sharp knife.
In a bowl, mash the avocados with a potato masher. Using a zester, grate a small clove of garlic and add it to the avocados. If you don't have a zester, a garlic press works too.
Add 3/4 tsp salt and 4 teaspoons freshly squeezed lime juice, snd 1 packed tsp chopped cilantro and mix it into the mashed avocados.
In the bowl of mashed avocados, mix in the tomatoes and drained onions. Then enjoy fresh.