Roast the tomatillos, chile(s) and garlic on a rimmed baking sheet 4 inches below a very hot broiler, until blotchy black and softening (they’ll be turning from lime green to olive), about 5 minutes. Flip them over and roast the other side. Cool, then transfer everything to a blender, including all the delicious juice the tomatillos have exuded during roasting. Add the cilantro and 1/4 cup water, then blend to a coarse puree. Scoop into a serving dish. Rinse the onion under cold water, then shake to remove excess moisture. Stir into the salsa and season with salt, usually 1/2 teaspoon.