Infuse the fish stock with 2-3 saffron strands.
Heat the olive oil in a large saucepan set over a medium heat and sweat the fennel for a few minutes with a little salt. Add the pastis and reduce a little, then pour in the infused fish stock.
Add the butternut squash balls, bring the soup to a simmer, and cook gently for 15 minutes until the butternut squash is tender.
Meanwhile, heat a large saucepan over a high heat for 1 minute, then add the mussels and white wine. Cover and let the steam cook them for 3-4 minutes. Strain the mussels and discard any that don't open. Add most of the mussels in their shells to the soup, but retain a few as a side garnish to the soup. Adjust the seasoning of the soup and ladle into soup bowls.
Arrange the reserved mussels in a side bowl and garnish both bowls with sprigs of thyme.