Preheat oven to broil with a rack about 4 inches below the heat source. Line a rimmed baking sheet with foil. Place the husked tomatillos and jalapeño on the baking sheet. Broil for 5 minutes; flip them over (tongs work best) and broil about 4-5 minutes more. The skins will begin to char and blister, but watch carefully to prevent then from completely burning.
While the tomatillos cool, chop your cilantro, onion, and garlic. Zest and juice your lime(s).
Place all the ingredients in a blender or food processor (including any juice from the roasted tomatillos) and pulse until mostly smooth and no big chunks remain.
The salsa will thicken after a few hours in the refrigerator. For creamy avocado salsa, let it cool down before blending in the avocado.
Source: Recipe slightly adapted from The Love and Lemons Cookbook