Preheat the broiler and place an oven rack in the top position. Line a baking sheet with foil. Broil the tomatillos, garlic and jalapeno (if using) on the baking sheet for 5 minutes. Keep an eye on the vegetables while they're broiling to make sure they don't burn. Turn the ingredients over and broil for another 5 minutes. The tomatillos should be brown in spots and will have started releasing juices. Remove from the oven.
Add the tomatillos, garlic, jalapeno, cucumber, shallot, cilantro, stock, lime juice, salt and water to a blender. Pit the avocado and scoop the flesh into the blender as well. Puree until very smooth.
Pour the soup into a large bowl, cover, and refrigerate for at least 2 hours before serving to allow to flavors to blend.