Note on chiles: This is one of the most versatile formulas to know, since you can go to practically any grocery store and find at least one variety of small hot dried chile. In a Mexican market (on either side of the border), the possibilities multiply quickly ? from the nuttiness of cascabel to the punch of arbol chiles, the peanutiness of piquin, and the smoky sweetness of red chipotles (morita). As a rough guide, 1/2 ounce dried chiles corresponds to 6 red chipotles (mortas), 4 tan chipotles, 16 arbols, 3 cascabels or 1/4 cup piquin.
Toasting and roasting. Preheat a broiler.
In an ungreased skillet over medium heat, toast the chiles, stirring for 1 minute, until they are very aromatic (some will have slightly darkened spots on them). Transfer to a bowl, cover with hot water and rehydrate for 30 minutes.
In the same skillet, roast the garlic, turning regularly, until soft and blotchy-dark in places, about 15 minutes. Cool and slip off the papery skin.
Roast the tomatillos on a baking sheet 4 inches below a very hot broiler until soft, even blackened in spots, about 5 minutes on each side. Cool, then transfer the contents of the baking sheet (including any juices) to a blender or a food processor.
Finishing the salsa: Drain the chiles and add to the tomatillos along with the garlic. Puree, then scrape into a serving dish. Stir in enough water to give a spoonable consistency, usually about 1/4 cup. Season with salt, usually 1 teaspoon, and the optional sugar. Refrigerated, the salsa keeps for several days.