How to Make It
In a large soup pot, heat the olive oil until shimmering. Add the onions and garlic, cover and cook over moderate heat, stirring occasionally, until softened, about 5 minutes. Add the stock, chestnuts, potatoes, thyme and cinnamon; season with salt and pepper. Cover and cook over moderate heat until the potatoes are very tender and broken up, about 15 minutes.
In a blender, puree the soup. Return the soup to the pot and stir in the sherry and lemon juice. Ladle into shallow bowls and serve.