Set an ungreased griddle or heavy skillet over medium heat. If using dried chiles, break off their stems. Toast the chiles a few at a time: lay on the hot surface, press flat for a few seconds with a metal spatula (they’ll crackle faintly and release their smoky aroma), then flip and press down to toast the other side. Transfer the toasted chiles to a bowl, cover with hot water and let rehydrate for 30 minutes, stirring regularly to insure even soaking. Pour off all the water and discard.
If using canned chiles, simply remove them from the adobo they’re packed in.
On a heavy, ungreased skillet or griddle over medium heat (you’ll already have it on if you’re using dried chiles), roast the unpeeled garlic, turning occasionally, until blackened in spots and soft, about 15 minutes. Cool, slip off the papery skins, then roughly chop.
Lay the tomatillos on a baking sheet and place about 4 inches below a very hot broiler. When the tomatillos blister, blacken and soften on one side, about 5 minutes, turn them over and roast the other side. Cool completely on the baking sheet.
Scrape the tomatillos (and any juices that have accumulated around them) into a food processor or blender and add the rehydrated or canned chiles and garlic. Pulse the machine until everything is thick and relatively smooth (detectable little bits will add textural interest).
Transfer your salsa to a serving bowl and stir in enough water, usually 3 to 4 tablespoons, to give the salsa an easily spoonable consistency. Taste and season with salt, plus a little sugar, if you want to soften the tangy edge.