To make a roux, melt the butter over a medium heat and stir in the flour. Keep stirring constantly for 3 minutes.
For the soup, add the mussel juice and the orange juice to the roux, stir well to combine and bring to the boil. Simmer for 5 minutes. Add the cream and the saffron and cook for another 5 minutes.
Sweat the vegetables for the garnish with a pinch of pepper and salt until tender.
Season the soup to taste with the orange zest, salt and pepper. Warm the mussel meat in the soup and serve hot with the garnish.
Notes
Use salt with moderation because the mussel juice is already salty.
A thick version of this soup can be used as a sauce for a pasta dish.
If you don't have a cocotte, use a large, heavy saucepan.