White onion soup with truffle oil

1) Melt the butter in a large saucepan over a low heat and add the onions, thyme and a heaped teaspoon of sea salt.
2) Sweat the onions for about 30 minutes until they are silky and soft. Stir frequently to prevent them colouring.
3) Pour over the white wine, turn the heat up and simmer until it is well-reduced.
4) Add the chicken stock, bring to a simmer and cook over a low heat for 15 minutes.
5) Remove the thyme stalks, and puree the soup with the crème fraiche and a grinding of black pepper.
6) Check the seasoning.
7) Return to a clean saucepan and gently reheat.
8) Serve in warmed soup bowls and top with a few drops of truffle oil to taste.