This salsa is even better when made ahead (up to 1 week). Refrigerate it until ready to serve, at room temperature.
Heat a large cast-iron skillet over medium-high heat. Add tomatillos; cook, turning a few times, until blistered in spots and beginning to soften, 10 to 12 minutes. Transfer to a bowl, cover with plastic wrap, and let steam 15 minutes.
Add garlic to skillet and cook over medium heat, turning a few times, until blistered and fragrant, 1 to 2 minutes. Transfer to a blender. Add chiles to skillet and cook, turning frequently, just until toasted and soft, 30 to 60 seconds. Transfer to blender with tomatillos and accumulated juices, salt, and 2 tablespoons water. Blend to consistency of a thin salsa, about 10 seconds.
