My good friend Pierre Koffmann is a cooking legend, and he makes a mean fish soup. The secret is to use pieces of fish and not just the bones or shells, as is so often the case, which gives it a much better flavour. Here, though, we’re using good-quality shop-bought fish stock. As well as aïoli or garlic mayonnaise, the classic garnishes are croutons and grated Gruyère cheese. Add them too if you want to feel as though you’re in Marseille!