Heat a cast-iron skillet over medium-low heat. Add chiles and garlic. Toast chiles and garlic, turning often, until lightly toasted and fragrant, 7 to 10 minutes; transfer as toasted to a medium bowl.
Increase heat to high. Add tomatillos and toast, turning occasionally, until blackened on all sides, about 10 minutes.
To make the salsa in a mortar, put toasted garlic and 1 tsp. sea or kosher salt in a deep mortar (molcajete) and pound to a coarse paste, pushing down with the pestle and then rocking it back and forth a few times before pounding again. Add toasted chiles and coarsely crush, then pound in tomatillos one at a time. To make in a blender, add all ingredients at once and pulse until coarsely puréed.