Melt the butter over medium heat in a large soup pot. Add the leeks and garlic and cook, stirring regularly, until soft and wilted, about 10 minutes. Adjust the heat as necessary so as not to brown.

Add the potatoes, stock, bay leaves, thyme, salt and pepper to pot and bring to a boil. Cover and turn the heat down to low. Simmer for 15 minutes, or until the potatoes are very soft.

Fish out the thyme sprig and bay leaves, then purée the soup with a hand-held immersion blender until smooth. (Alternatively, use a standard blender to purée the soup in batches; see note.) Add the heavy cream and bring to a simmer. Taste and adjust seasoning with salt and pepper. If soup is too thin, simmer until thickened. If its too thick, add water or stock to thin it out. Garnish with fresh herbs if desired.

Note: If using a standard blender to purée the soup: be sure not to fill the jar more than halfway; leave the hole in the lid open and cover loosely with a dishtowel to allow the heat to escape; and pour blended soup into a clean pot.