Cook onions and a little of the water in large stock pot over medium-high heat until the onions are translucent.
Add curry, carrots, sweet potato, and remaining water. Add more water if needed to cover the ingredients, but not too much or your soup will be thin. Cook until vegetables are soft, 20 minutes.
Add the apples and maple syrup, if using, and let everything cook together for another 5 while you prepare your blender or immersion blender.
Blend either in the pot with an immersion blender, or transfer to an upright blender in batches. Soup should be thick and smooth.