Peel the potato and cut it in six pieces. Cook in plenty of boiling lightly salted water for 15–20 minutes, as if you were preparing mashed potatoes. When it is cooked, put the pieces to drain in the mouli-légumes. Throw out the cooking water, clean and dry the saucepan and place the mouli-légumes over it. Sieve the potato and, immediately stir in 20g (¾oz) butter. Work well with a wooden spatula in order to obtain a very smooth puree. Set on one side in a bowl.
While the potato is cooking, wash the leek and the celery and peel the carrot. Cut all three into large julienne strips and cook in 1litre (1¾pints) boiling salted water. When the vegetables are cooked, but still slightly firm, put them to drain in a colander.
If you are using French beans, top and tail them and cut into short lengths. Whether you are using beans or petits pois, plunge them into 1 litre (1¾ pints) boiling salted water and cook them uncovered (you must never cover green vegetables during cooking) for 5 minutes for petits pois and 10 minutes for French beans. Drain them in the colander with the other vegetables (2).
Scrape and clean the mussels and wash them in several waters. Place them in a large saucepan with the chopped shallot and dry white wine. Cover the pan and cook briskly, shaking the pan from time to time to move the mussels about. Cook for 5–10 minutes or until all the mussels are open. Remove them with a slotted spoon and allow to cool on a plate. Keep the cooking liquid. When the mussels have cooled, shell them and keep them on one side.
Cut the fillets of sole into strips the length of your little finger. Strain the mussel liquid (4) through a fine strainer into a saucepan, keeping back the last few tablespoons, which may be sandy. Reduce this liquid until there is no more than 3tablespoons left. Add the cream and the fillets of sole and bring briefly to the boil. Take out the strips of sole with a slotted spoon and put them in the colander with the vegetables and the mussels.
Bring the liquid in which the sole has cooked back to the boil then add the potato purée (1), a spoonful at a time, whisking all the time. Whisk till the sauce is perfectly smooth (or alternatively purée it in the liquidiser to give an even more velvety texture).
When you want to eat the dish turn the contents of the colander (vegetables, mussels and sole) into the sauce. Taste and season with pepper – and salt if the mussels have not provided enough. Add a pinch of saffron and the chopped parsley. Bring up to boiling point and serve in deep, heated plates.
dry white wine such as Sauvignon, Vin de Palette, Meursault