The secret of this rich, warming soup lies in the slow cooking of the onions and leeks, in just a little butter and oil, until lightly caramelised. Thinly sliced potatoes add extra carbohydrate and help to thicken the stock. Complemented by crunchy cheese and walnut-topped croûtes, it makes a satisfying lunch or supper dish.
For a vegetarian soup, use a good vegetable stock instead of beef stock and stir in ½ tsp yeast extract for a richer flavour and colour. For the croûtes, use Emmenthal cheese as Gruyère is not suitable for strict vegetarians.
Sherry vinegar is matured in wooden barrels by methods similar to those used for sherry itself. It has a rich, mellow flavour but if unavailable you can use red wine vinegar instead.