Place the first 5 ingredients in your slow cooker and whisk until the flour is completely dissolved. Mix the squash, apples and ginger. Cover and cook on the low setting for 6 hours, or until the squash is very tender.
Puree the soup in a food processor or blender, or in the pot of the slow cooker with a hand (immersion) blender. Taste the soup and season with salt, if needed. If desired, garnish bowls of the soup with a dollop yogurt and a sprig of mint.
Notes: Cleaned, ready peel banana squash is available in our produce department. Once cooled to room temperature, this soup freezes well. This soup was tested in a 6 litre capacity slow cooker.