Remove the husks from the tomatillos and drop them whole into a pot of boiling water for 5 minutes to soften them. Using a slotted spoon or tongs, transfer them from the pot to the blender.
Remove the pit and scoop out the avocado flesh. Add it to the blender along with the garlic, onion, peppers, lime juice, and sour cream.
Blend until you reach desired consistency.
Add cilantro, cayenne, and a large pinch of salt, pulsing once or twice to combine.
Leftovers will keep several days in the fridge. Yields 2 cups.