Heat a heavy frying pan over medium-high heat. Cook tomatillos and serranos, turning, until blistered and charred all over, 15 to 25 minutes; transfer to a plate as done.
Let chiles cool, then discard stems and peel off a little of the blackened skins. Put tomatillos, chiles, garlic, onion, 2 tsp. salt, and 6 tbsp. water in a blender. Whirl until fairly smooth, adding 2 tbsp. more water if salsa looks too thick. Add avocado and cilantro and whirl just to blend. Season with more salt to taste.
Make ahead: Up to 1 day, chilled.