Put the stock in a saucepan with the saffron, garlic, sage, thyme and half the fennel. Bring to the boil, lower the heat and simmer for 30 minutes. Keep the pan covered. Remove the garlic and press to extract the pulp from the skins. Discard the herbs. Poach the fish in the broth for 5 minutes. Keep warm.
In a food processor or a mortar, work the garlic pulp and egg yolks to a paste. Whisk in a ladleful of the fish broth and stir in the cream. Pour the emulsion back into the pan with the fish.
Taste for seasoning; warm over a low heat, stirring. Do not overheat or it will curdle. Chop the remaining fennel, scatter it over the soup and serve with toasted pain de campagne and, if you like, aïoli.