Dice the potato, leek, onion, celery and fennel into small pieces. Place a large saucepan on the heat, add the butter and sweat the vegetables with the lemon thyme, bay leaf, saffron and garlic. When cooked, add the mussels and the white wine and cover the pan with a lid. As soon as the mussels open, remove them from the pan. Remove the flesh from the shells and set aside. Discard the bay leaf and lemon thyme. Add the fish stock to the pan and bring to the boil, then add the cream, reserved mussel meat and chervil. Blend, season and pass through a sieve.
To make the rouille foam, combine the ingredients in a small saucepan and heat gently. Use a hand blender to froth the mixture to a light foam.
To make the sea urchin butter, whip the butter until pale. Put the sea urchin meat, cayenne pepper and lemon juice in a blender with the chillies and garlic and mix to a fine cream. Fold this mixture into the whipped butter. Fold in the chopped chervil. Roll in waxed paper into a log about the circumference of a 20-cent piece and refrigerate.
Just before serving, heat the soup gently in a saucepan, making sure it does not boil. Pour it into the sea urchin shells (or bowls if you don’t have the shells). Place a disc of butter on top and then arrange the rouille foam around the outer edge. This is lovely to eat with a long stick of sourdough bread fried in olive oil with some minced garlic and parsley.