Place the potatoes, onions, chopped lemongrass and stock in a large heavy-based saucepan and bring it to the boil. Reduce the heat, cover and simmer for 15-20 minutes until the potatoes are just cooked.
Stir in the sorrel, spinach, spring onions, chilli, garlic, sugar or honey and a pinch of salt. Bring the soup to the boil and cook for 1 minute.
Remove the soup from the heat and add lemon juice to taste. Serve it hot or chilled, as you prefer.
Variation: if sorrel is not in season, use another strong-flavoured leaf such as rocket or watercress.