Spray a pan with some low-calorie cooking spray and set on a medium heat. Add the onion and fry for 5 minutes, until softening and turning lightly golden.
Stir in the curry powder, then add the parsnips and apple to the pan. Cook for a few more minutes, until the curry powder becomes fragrant.
Add the stock cubes to the water and stir until dissolved. Pour into the pan and bring to the boil.
Cover with a lid and lower the heat, then simmer for 25 minutes — until the parsnips are soft.
Remove the pan from the heat and, using a stick blender or food processor, blitz until smooth.
If the soup is too thick, add some more water until it reaches the desired consistency.
If you want, feel free to serve with a few drops of hot sauce.