About this recipe: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.
15g unsalted butter
1 tbsp olive oil
3 large onions, halved and thinly sliced
2 leeks, halved lengthways and thinly sliced
1 garlic clove, crushed
2 tsp fresh thyme leaves
½ tsp caster sugar
1 tbsp sherry vinegar
3 small potatoes, about 300g in total
1.4 litres beef stock (well-flavoured)
4 fresh chives, snipped
3 thick slices wholemeal bread
1 garlic clove, cut in half
2 tsp French mustard
75g Gruyère cheese, coarsely grated
50g walnuts, roughly chopped
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Method Prep:10min › Cook:1hr › Ready in:1hr10min
Heat the butter with the oil in a large saucepan. Add the onions and leeks and 1 tsp water. Cook over a medium heat for 7–8 minutes, stirring frequently until they begin to soften. Turn down the heat to very low, cover and cook gently for 20–25 minutes, stirring occasionally. Increase the heat slightly, add the garlic, thyme and sugar and cook, stirring, for 1 minute. Then add the vinegar and continue cooking, stirring frequently, for 3–4 minutes until the onions and leeks are light golden.
Peel the potatoes, cut in half lengthways and thinly slice. Add to the onion mixture with the stock. Bring to the boil, then reduce the heat, cover and cook for 15–20 minutes until the potatoes are tender and just beginning to break down and thicken the soup. Season to taste. Preheat the grill to medium.
Meanwhile, prepare the croûtes. Trim the crusts off the bread and cut each slice into 4 triangles. Place on the rack of the grill pan and toast very lightly on both sides. Gently rub each croÛte with the cut surface of the garlic, then spread very thinly with mustard. Top with the cheese and walnuts, pressing down gently.
Pop the cheese-topped croûtes back under the grill and toast for 2–3 minutes until the cheese is just starting to bubble and brown and the walnuts are lightly toasted.
Sprinkle the soup with snipped chives, then serve with the croûtes popped on top.
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.