For the soup, heat the butter in a pan over a medium heat and gently fry the onion for 3-4 minutes, until softened.
Add the potato and broccoli to the pan and stir. Add the chicken stock and bring the mixture to a simmer. Simmer for 8-10 minutes, or until the vegetables are tender, then stir in the cream.
Blend the mixture with a stick blender until smooth.
For the pistou, blend all the pistou ingredients in a food processor until smooth.
Serve the soup in a bowl with the pistou and cream drizzled over the top. Garnish with the chopped parsley and chervil.
