Peel and finely chop the onion. Warm a pan for 1 min, then add 1 tbsp oil and the onion. Season. Cover and turn the heat down low. Sweat for 8 mins till soft.
While the onion sweats, peel and crush or grate the garlic. Halve the chilli and flick out the seeds. Finely chop it. Break the broccoli into small florets, and roughly chop the stalk, discarding the tough end.
Stir the garlic and chilli into the onion. Add the broccoli florets and stalk. Pour in the coconut milk and 600ml boiling water. Bring to the boil, uncovered (this helps keep the broccoli green), and simmer for 5 mins till the broccoli is very tender.
While the soup simmers, tip the sunflower seeds into a dry frying pan. Toast them over a medium heat for 2 mins, shaking the pan now and then, till the seeds are golden and toasted. Tip into a bowl.
Add half the sunflower seeds to the soup. Ladle it into a blender and blitz till smooth, or use a hand-held blender in the pan. Taste the soup and season if needed. Ladle it into warm bowls. Top with the remaining seeds and serve straight away.