Step 1In a heavy-bottom saucepan, combine the water, chiles, garlic, tomatillos and cilantro. Bring the mixture to a simmer, loosely cover, and cook until the chiles and tomatillos are softened, about 5 minutes. Remove from heat.
Step 2Drain, reserving the water, and place the ingredients into a blender. Add the avocado and salt. Carefully blend at low speed, adding water as needed, until the mixture is pureed to form a thin sauce. Taste and adjust the seasoning if desired. The sauce will keep, covered and refrigerated, up to one week.