On a heavy, ungreased skillet over medium heat, roast the unpeeled garlic, turning occasionally, until blackened in spots and soft, 10 to 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 garlic, chipotle and salt. Pulse until everything is coarsely pureed. Transfer to a serving bowl along with enough water (usually 3 to 4 tablespoons) to give the salsa an easily spoonable consistency.