Wash the mussels very well with water and remove any barnacles.

In a large cooking pot, put the mussels in water and heat. Wait until they open and stop the heat. Be careful not to overcook the mussels. Take the mussels out, remove shells and set aside.

Keep the cooking water.

Chop the onion, leeks and garlic. Saute onion and leek in olive oil in a pan with high edges. Reduce the heat.

Add tomato paste and garlic, and stir. Wet the mixture with the reserved cooking water from the mussels, add fennel, bay leaf, and a pinch of saffron, season (not to much salt) and bring to boil.

Meanwhile, chop a handful of mussels and add them to the broth. Allow the soup to absorb the flavour and aroma from the mussels (about 10 minutes).

Remove the bread from the soaking liquid and discard the milk.

Tear the bread into large pieces and pound with a garlic clove in a mortar with a pestle until you get a paste.

Stir in egg yolk, then add some olive oil, stirring constantly to build the rouille.

Add more olive oil, then add 200ml (6.5 oz) of the mussel soup and stir again.

When the paste becomes the consistency of a mayonnaise, season with salt, pepper and a pinch of cayenne pepper powder.

Slice the bread into thin rounds, rub with garlic and grill lightly to create crostini.

Pour the soup into a soup tureen after placing the rest of the mussels. Serve in soup plates.

Put the grilled croutons rubbed with garlic, rouille and grated cheese in separate bowls. Each guest may add croutons, rouille and cheese to their taste.