Roast ingredients: Spread the tomatillos, unpeeled garlic and chiles on a rimmed baking sheet and roast about 4 inches below a hot broiler until the tomatillos and chiles are blotchy brown on one side, about 6 minutes, then turn everything over and roast the other side. The garlic should feel soft.
Cool and blend: Cool until handleable, slip the peels off the garlic, then scrape everything (including any juices) into a blender or food processor and pulse until you have pureed it to your liking.
Finish and season: Gather the cilantro into a tight bundle and thinly slice across it until you run out of leaves. Chop the onion into about ¼-inch pieces. Rinse in a strainer under cold water to get rid of the unpleasant 'oniony' flavor. Add to the salsa along with the cilantro. If the salsa is too thick, stir in water a tablespoon at a time until it seems right. Taste and season with salt.
