This classic version is made with good-quality beef stock, caramelised onions and topped with oozing Gruyère cheese toasts. Gruyère is the key to getting the bubbling crust because it’s rich, smooth and melts easily, but you can also use blue cheese, Comte or aged Cheddar. Sometimes I add a splash of sherry or a cup of stout to the stock – both bring wonderful and very different flavours. It really is the perfect soup supper – rich in flavour, warming and utterly delicious.