If you’ve ever tried potato leek soup and found it to have a gluey texture, it’s because the potatoes released too much starch from being over handled. This is unfortunately easy to do with a food processor or blender (heck, it can even happen when making mashed potatoes, which is why I avoid using an electric mixer). I’ve never run into this issue when pureeing with an immersion blender, though. If you do have to use a blender or food processor, puree the soup in brief bursts and check it frequently. You can alternatively just mash the potatoes or push them through a ricer, but I feel that doesn’t get me the consistency I want (it’s not thin enough).