On a small baking dish or baking sheet lined with aluminum foil, place the tomatillos, garlic, onion slice and chile. Place under the broiler for 8 to 10 minutes, flipping the ingredients once in between, until completely charred and mushy. Watch out for the garlic as you may need to remove it a few minutes before the rest of the ingredients. When cool enough to handle, peel the husk off the garlic cloves and remove stem from the chile.