Preheat the broiler. Line a baking sheet with aluminum foil. Place the tomatillos in a single layer on the baking sheet. Broil until darkened and softened on one side, about 10 minutes. Turn tomatillos over and continue to broil the other side for approximately 10 more minutes. Let cool completely. Place the roasted tomatillos and any juice that has accumulated in a food processor or blender.

Add the chili peppers and garlic, and pulse a few times to puree. Add onion, cilantro, salt, and sugar to the tomatillo mixture in the food processor. Pulse a few more times until salsa has reached the desired consistency and chunkiness. If the salsa is too thick, add up to ¼ cup of water.