Set the oven or toaster oven to broil (alternatively, you can preheat the oven to 500degreesF) and preheat. If you're using the oven broiler, position the rack 8 inches from the heat source.
Put the tomatillos on a foil-lined baking pan and roast in the oven, turning them over once halfway through, until their tops and bottoms have blackened and the tomatillos are a khaki-green color and cooked to the core, 20 to 30 minutes. Let them cool to room temperature.
Meanwhile, heat a comal, griddle, or heavy skillet over medium-low heat and toast the chiles and roast the garlic on the comal, turning them over frequently until the chiles have puffed up and are blistered in spots, 3 to 5 minutes, and the garlic is tender and golden brown with some blackened spots, 8 to 10 minutes.
Put the roasted tomatillos, chiles, and garlic in the blender jar with the water and salt and blend until smooth (the tomatillo seeds will still be visible). Season to taste with additional salt.
This salsa keeps in the refrigerator for up to five days.
Reprinted with permission from Truly Mexican by Roberto Santibanez with JJ Goode, copyright (C) 2011. Published by Wiley.