The roasted tomatillos will have the most pronounced flavor the first day you make this salsa.
Heat the broiler. Place a heavy, ungreased, small skillet over medium heat. When skillet becomes hot, add the garlic. Cook, turning occasionally, until the garlic is blackened in spots and soft, about 20 minutes. When the garlic is cool enough to handle, peel, and toughly chop.
Place tomatillos on a baking sheet, and place baking sheet 4 inches below the heating element. Broil until blistered, blackened, and softened, about 5 minutes per side. Remove the baking sheet from oven, and let tomatillos cool completely.
Scrape tomatillos and juices into the bowl of a food processor or into the jar of a blender. Add the garlic. Pulse until a coarse puree forms. Stir in the chiles and 3 to 4 tablespoons water, until a spoonable consistency forms. Add the salt and sugar; stir to combine. Store in an airtight container, refrigerated, up to 1 week.