Sweat all the chopped vegetables, garlic, herbs and saffron in the olive oil and butter for a few minutes. Add the red mullet fillets, the Noilly Pratt and white wine and allow to cook and reduce for 5-6 minutes.
Pour on the fish stock and tomato puree. Bring the soup to a simmer and cook for 40 minutes.
Puree the soup in a food processor or liquidiser and push through a sieve. Season with salt and pepper. Serve with the Rouille toasts.