I made a zucchini (summer squash, really) soup the other day and followed the technique I use for most puréed vegetable soups, including the asparagus, carrot and potato-leek soups I've previously written about.
However, in this case, I messed up and added too much water to the pot after building the soup's base (sweated—not browned—onions, garlic and various types of zucchini). After 30 minutes of simmering and after using a hand-held immersion blender to purée the mixture, I was left with a watery mess that was less than palatable.
What to do to thicken the soup? I went to the market and bought a handful of small Yukon Gold potatoes, which I cut and boiled in water until they were soft. I added the potatoes to the zucchini soup and used the immersion blender again. As hoped, the cooked potatoes thickened the mixture as needed. (Thinking about it, I could have cooked the potatoes in the zucchini soup.)
Just to be safe, I think I'll always have potatoes on hand whenever I make a puréed vegetable soup. Either that or add the correct amount of water.