Roughly chop half of the tomatillos and scoop them into a food processor with the cilantro and green chiles. Measure in 1/4 cup water and process to a slushy, coarse puree. Roughly chop half the avocado, add it to the processor and pulse until it is incorporated into the salsa. Scrape into a serving dish. Scoop the onion into a small strainer and rinse under cold water. Add to the salsa. Finely chop the remaining tomatillos and add them, too. Finally, chop the remaining avocado into 1/4-inch pieces and stir them into the salsa. Taste and season with salt, usually about 3/4 teaspoon.
Alternative Method: Put all the tomatillos (quartered), cilantro and chiles through a meat grinder to chop them (no water necessary), then stir in finely chopped avocado and rinsed onion.