This is from http://www.101cookbooks.com/archives/000171.html.
Add the tomatillos, serranos, onion, and garlic to the vegetable stock in a large pot. Bring to a boil and immediately reduce to a simmer. Cook over moderate heat for 20 minutes or until tomatillos are tender and losing their form.
Puree the vegetable/stock mixture well with a blender (my hand-held blender works great here). You want to puree it really well. Push through a strainer into a bowl (optional) to remove tomatillo seeds and skins and allow to cool on the counter. Stir in 1/4 cup of the lima juice and place in the refrigerator. Chill well, at least a couple hours or overnight.
When the soup base is completely chilled, puree the avocado thoroughly with a blender along with the cilantro and a couple splashes of the lima juice. Stir into the chilled soup base. Taste to see if you want to add more lima juice - you should be able to taste and smell the lima/lime flavor, but it certainly shouldn’t be mouth-puckering. Whisk in 1/2 cup crema. Salt and pepper to taste then return to the refrigerator until ready to serve.
Pour soup into bowls, add a swirl of crema, a spoonful of salsa, and a sprinkling of tortilla strips to serve.