To make the soup, melt the butter in a saucepan over a medium heat. Add the bay leaf and garlic, followed by the onions; cook and stir and allow to soften, about 6 minutes.
Add cubed potato and stir, then pour in enough stock to cover potatoes, simmer until soft, about 15 minutes.
Add milk and bring to a simmer. Add half of the spinach to pan, add salt, pepper, and nutmeg. Remove from heat. Remove bay leaf.
Transfer liquid into a blender, add remaining uncooked spinach and blend. Pour into a clean pan and reheat as necessary.
To make truffle croutons, take a few slices of bread and use a biscuit cutter to cut into circles. Fry in oil until crisp and golden. Remove from the heat and place on kitchen paper to drain; drizzle with truffle oil (less is more with truffle oil as it is quite potent).
Ladle the soup into warmed bowls then add a few croutons on top. Serve immediately.
Adding half of the spinach towards end of the cook and the remaining whilst blending allows for that amazingly vivid colour.