Remove the husks from the tomatillos, wash well and rinse, cut in half, place on a rimmed baking pan that is broiler safe. To that pan add the garlic cloves with the skins left on and the washed serrano chiles.
Broil for 4 minutes or until lightly browned, turn everything over and broil another 4 minutes.
Cut ends off the serrano chiles and remove the garlic from the skins.
Place the tomatillos along with all the juices into a blender or food processor along with the garlic cloves and 1 or 2 chiles along with the seeds or not depending on how hot the chiles are and how much heat you want the salsa to have, add the water and process – add the onion, cilantro and salt and pepper if using, process until desired consistency is achieved. If its too thick add a little more water. It does thicken as it cools but becomes more loose as it warms up.