Use the metal blade of a food processor. Drop the garlic through the feed tube with the motor running. Add the onion and pulse to chop. Place in a strainer and rinse with cool water to remove the bitter milky liquid. Remove the husks from the tomatillos. Rinse. Quarter. Place the tomatillos and chili peppers in the workbowl of the food processor. Pulse to mince finely. Add the cilantro. Pulse to combine. Stir the onions and tomatillos together. Add sugar and salt to taste. Keeps less than 8 hours. Add water if the salsa thickens before being used.