At Vidal and Barbara Mozqueda's backyard parties in San Francisco, Barbara is in charge of making the salsas. This one, medium-hot and with a nice char on the tomatillos and chiles, is always a hit with Vidal's Carne Asada with Nopales and Spring Onions.
This recipe goes with: Carne Asada with Nopales and Green Onions
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.
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.