Halve the orange and squeeze the juice. Mix the orange juice with cayenne pepper and pour over the prawns.
Mix the tomato pieces with the coconut milk and stock in a pan. Bring to the boil and simmer gently for 5 minutes.
Wash the spring onions and cut diagonally into thin rings.
Add the prawns and orange juice to the soup and heat through. Season the soup with salt and cayenne pepper.
Serve the soup in deep plates or soup bowls. Sprinkle with the spring onions to garnish.