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
