Grill poblano directly on gas flame or on a grill pan until blackened, turning frequently, 12 to 15 minutes. Transfer to a paper towel; let cool. Peel, seed, and remove stem; set aside.
In the bowl of a food processor fitted with the metal blade, combine tomatillos, onion, poblano, cilantro, and salt. Pulse until coarsely chopped, about 30 seconds. Can be made ahead up to this point and stored refrigerated for up to 3 days.
Transfer to a medium bowl. Stir in avocado. Taste and adjust for seasoning. Serve immediately.