Melt the butter in a heavy saucepan, add the onions and cook gently over a medium heat for 15 minutes, or until soft and lightly browned.
Add the cider and cider vinegar and boil until reduced by about a half.
Add the thyme, bay leaf, chicken stock, potato and a little salt. Bring to a simmer, and cook for about 15 minutes over a low heat.
Remove the bay leaf and purée the soup with a hand blender until smooth. Season to taste with salt and freshly ground black pepper.
To make the Stilton butter, mix all the ingredients together in a bowl until they are well blended.
To serve, ladle the hot soup into warm bowls. Add 1 tbsp of the Stilton butter to each bowl and garnish with fried onions and chervil
