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.
3 portions vegetables
Sherry vinegar is matured in wooden barrels by methods similar to those used for sherry itself. It has a rich, mellow flavour that makes it popular in cooking. If sherry vinegar is unavailable, you can use red wine vinegar instead.
For a vegetarian soup, use a good vegetable stock instead of beef stock and stir in ½ teaspoon yeast extract for a richer flavour and colour. For the croutes, use a Swiss-style non-animal rennet cheese as gruyère is not suitable for strict vegetarians.