Melt the butter in a large, wide pan over a medium heat and cook the leeks for 5 mins until lightly brown.

Pour in the vermouth, if using, and simmer for 1 min, then stir in the stock, butter beans and thyme. Bring to a simmer, cover and cook for 5 mins. Remove the lid and cook for a further 8-10 mins, until the leeks are tender and most of the stock has evaporated. Turn off the heat, stir in the crème fraîche and serve scattered with the chopped parsley.