Preheat broiler. Line rimmed baking sheet with foil. Place tomatillos on prepared baking sheet. Broil until tomatillos are just blackened in spots and tender, about 8 minutes per side.
Combine onion, cilantro, chiles, and lime juice in large bowl. Add roasted tomatillos and any juices from baking sheet to onion mixture. Using fork, mash coarsely. Add avocados and mash with fork until mixture is very coarsely pureed and some chunks remain. Season guacamole to taste with salt. (Can be made 4 hours ahead. Cover and chill.)
