Cut each avocado in half by rotating the knife around the pit. Remove each pit and using a spoon, scoop the entire fruit out from the skin of each half into a bowl. Mash the avo with a wooden spoon or a fork, you don’t want it perfectly smooth, just a rough mash. Add the garlic, chilli, lemon juice, ¾ of the onion, ¾ of the tomato and ¾ of the coriander.. Mix together well and season to taste with salt and pepper. Combine the remaining onion, tomato and coriander and sprinkle on top. Serve with stone-ground corn nachos.