Char chiles in broiler or over a gas flame until blackened all over. Place in a medium bowl; cover with plastic. Let steam for 15 minutes. Meanwhile, heat a large dry cast-iron skillet over medium-high heat. Toast garlic until tender and skins are charred, about 8 minutes. Let cool. Wearing gloves, peel chiles; discard stems. Peel garlic cloves. Pulse chiles, garlic, lime juice, and 2 tsp. salt in a blender until a coarse purée forms. Season with salt.