Quarter the tomatillos and put them in a food processor. Add lime juice, garlic and chiles. Pulse the processor until you have the desired consistency. I like mine a little bit chunky, but not too much. Pour the sauce into a sauce pan and bring it to a soft boil.
Add the shredded chicken and let it simmer. Add salt and pepper to taste.
Put the sour cream in a bowl, and add just enough milk until it's a smooth sauce that's pourable.
Load up bowls with the chicken mixture. Top with avocado, cilantro, sour cream sauce and corn chips.