Heat a heavy cast-iron skillet or griddle over medium-high heat.
Place tomatillos in pan and toast until charred on 1 side, about 10 minutes for a medium or large tomatillo. The color in the middle should be fading from pale green to olive. Turn tomatillos over and continue to grill until charred on the other side, about 10 minutes, but not for so long that they burst.
Place chile(s), garlic cloves and onion slices in skillet and toast, turning often, until chile is lightly charred and garlic is charred in spots and softened. The onion should be lightly colored on both sides but not charred black (that will make it bitter). Remove from heat and transfer vegetables to a blender.
Add remaining ingredients to blender and blend until smooth. Taste, adjust seasoning, and serve.
