1. Put onion, chilies, cilantro and salt into a molcajete, and crush to a paste. (Or puree in a blender and transfer to a mixing bowl).
2. Cut avocados in half, remove pit, and squeeze out the flesh. Mash avocado into onion-serrano base and mix well. Add yogurt, stir to combine. Stir in ½ cup of tomatoes.
3. Sprinkle the surface with garnishes: tomato, onion and finely chopped cilantro. Serve immediately.