Sorrel and potato soup

SOURCE This recipe is adapted from one for cawl suran (which is the Welsh for 'sorrel soup') in Celtic Cuisine (G. Davies, Graffeg 2008).
Serves 4 as main course, 6 as starter
Time (approx): preparation 30m cooking 30m

3 tbsp cooking oil
250g onion
600g potatoes
1 clove (10g) garlic
120g fresh sorrel leaves
2L vegetable stock*
Salt and pepper to taste
Yoghurt or cream to serve (optional)
* Or 2L water plus 4 tsp of good-quality bouillon powder or 2 stock cubes.
Chopping board
Knife with a sharp blade
Large saucepan (min. 2.5L)
Kettle or other means of boiling water
Measuring jug
Blender or food processor
A hob or other direct heat source; space to chop.
Advance preparation
None required.
Preparing the Vegetables
Peel and chop the onion.
Wash, peel and chop the potatoes in (approx) 1cm cubes.
Peel and crush the garlic.
Remove any thick stems or ribs from the sorrel leaves, and wash the leaves.
Cooking the meal
Heat the oil in a large pan and sweat the vegetables and the garlic over a low heat for about 5 minutes or until the onion is translucent.
If you are not using pre-made vegetable stock, boil enough water to get 2 litres, put the bouillon or stock cubes a jug or bowl, add the hot water and mix well.
Add the sorrel leaves to the vegetables and cook gently until leaves have wilted.
Add the stock to the sweated vegetables and sorrel, bring to the boil then turn down and keep simmering on a low heat until the vegetables are cooked through (about 25 minutes).
When the vegetables are cooked, liquidise the soup using the blender or food processor. If using a processor with a jug (rather than a hand-held blender), let the soup cool a little beforehand.
If preferred, you can add a little yoghurt or cream before serving, and salt and pepper to taste after serving.