This is a Rolls-Royce version of moules marinières that’s simple to make. The mussels are cooked with wine, taken out of their shells and served in a velvety purée made with potatoes, leeks and onions. The soup’s beautiful buttercup-yellow colour comes from saffron, and the flavour is enriched with fish stock. Serve it with crusty bread and butter. To make this soup into a complete meal, double the quantity of mussels.