Cut cucumber into smaller pieces and scoop out the avocado flesh. Add everything to a blender and blend until creamy and smooth. Adjust thickness with more water if necessary – and season to your taste.
Heat a skillet to medium heat. Roast the sunflower seeds together with salt until golden. Transfer to a plate until cooled.
Serve the soup straight away – you could blend in a few ice cubes to make it ice cold, or store it in the fridge until it is time to serve. Sprinkle with a sunflower seeds and a few fresh mint/parsley leaves.