1/2 c sour cream
1 to 1-1/2 c ice water
1 Tbs good soy sauce
1/2 to 3/4 tsp salt
2 medium globe tomatoes or about 2/3 of a pint of grape tomatoes
2 to 3 scallions
Puree the avocado and sour cream along with a little of the water in a blender or food processor. Add the soy sauce, salt, and the rest of the water to your desired consistency. Taste for salt; it will need to be quite salty for the tomatoes. Dice the tomatoes and add to the soup; cut the scallions lengthwise and then slice thinly and add to the soup.
You want this well chilled, but it should be served soon after making it--I suggest chilling it over ice water so it'll be quick, or I imagine you could use a few ice cubes in place of some of the water if you're using a blender. If you make this fairly thick, there will be enough air incorporated into the soup that even if you press plastic wrap onto the surface it will discolor and get old tasting if it sits a while; this is less of a problem if the soup is thin.
The soy sauce is basically to add a rich/meaty character--I imagine you could use a mix of chicken (or vegetable) stock and water instead of soy sauce and water. One-third stock, two-thirds water would be my guess.
I've made this with frozen avocados when fresh weren't available and it works, though there's enough citric & ascorbic acid added to them (to prevent browning) to be tasteable.