Cook the onion gently in the olive oil or
butter for 5-10 mins, until soft. Add the
potato and cook for 5 mins, then add
the courgettes, rocket, stock and plenty
of seasoning. Bring to a simmer and cook
for 5 mins, until the courgettes feel soft.
Whizz the soup in a blender, or use a
hand blender, until smooth. Return to
the pan and keep warm.