Place the tomatillos, chiles, garlic cloves, and onion in a dry, cast iron pan. Toast, turning occasionally over medium-high heat until the husks of the tomatillos have blackened and their skins turn translucent. The goal is to soften the tomatillos by blackening the skin without allowing them to split. Remove from pan, and allow to cool slightly.
Remove the husks from the tomatillos and the stems from the peppers. Place into the bowl of a food processor with the cilantro and salt to taste; process to desired consistency. Pour the salsa into a saucepan, and cook over medium heat for about 5 minutes to mellow the flavors and remove the raw taste.