Mussel and sorrel soup

Starter: Serves 4. Total time: 30 minutes

Despite a centuries-old dalliance with sorrel, it is still easier to find this herb in the wild than in a shop. In The Englishman's Flora, Geoffrey Grigson reels off a local name for every county. Bread and cheese in Devon, cuckoo's meat in Cheshire and Tom Thumb's thousand fingers in Kent, none of which are as logical as the "sour dabs" that children over the ages have nibbled on their way to school. Sorrel was traditionally used as a souring agent, and as such has the same affinity with fish as lemon. I am equally sold on the way it melts down to an exquisitely silky mass that challenges the tenderness of spinach.

2kg mussels
30g unsalted butter
80g sorrel (stalks trimmed), sliced
150ml vermouth
220ml water
150g crème fraiche
2 heaped tsp beurre manie (equal quantities of plain flour and unsalted butter, blended)
sea salt, black pepper

To clean the mussels, soak them in a large sink of cold water. Pull off the beards and scrape off any barnacles, discarding those that are broken or that do not close when sharply tapped. Rinse in cold water, then place in a large saucepan. Cover and cook over a high heat for 4-5 minutes until the mussels have steamed open, shaking the pan or stirring them halfway through. Remove the lid and, once the mussels are cool enough to handle, shell three quarters of them and reserve in a bowl, together with the remaining shell-on mussels.

Melt the butter in a saucepan over a medium heat, add the sorrel and stir until it changes from a bright to a dull green. Pour in the vermouth, bring to a simmer and cook to reduce by half. Add the mussel liquor, discarding the last, gritty bit, then add the water. Bring to a simmer, stir in the crème fraiche and then the beurre manie, and keep stirring until this melts. Simmer the soup for 5 minutes, then season with black pepper, and salt if required. It can be prepared to this point in advance.

Just before eating, bring the soup back to a simmer and add the mussels to heat through. Serve in warm bowls.