Add green tomatillos, garlic clove, 1/3 of the onion and chili to a medium saucepan over medium/low heat. Add enough water just to cover, and bring to a boil. Lower the heat to low and simmer for 6 to 7 minutes.
Drain cooked salsa ingredients and place in a food processor, with remaining raw onion, fresh cilantro, avocado and ½ teaspoon salt to start (do not add extra water). Process until smooth.
Serve straight away, or store in an airtight container in the fridge for up to three days.