This is a light, creamy soup with a delicate flavor. The addition of celery seeds helps emphasize the mild celery taste. For a more complex flavor, stir in creamy blue cheese or Stilton to taste after the bisque has been puréed.
Melt the butter in a large saucepan over low heat. Add the sliced celery, leeks, and potatoes, and stir to blend. Press a round of parchment or wax paper directly atop the vegetables and cook the mixture slowly over low heat for 20 minutes or until they're soft, stirring once or twice to make sure the vegetables don't brown.
Add the milk, stock, and celery seeds, and bring to the start of a simmer over medium heat. Reduce the heat to low, cover the pan with a lid, and cook slowly until the vegetables are extremely tender, about 30 minutes.
In a blender or with an immersion blender, purée the soup until smooth, and season to taste with salt and pepper. Divide among heated bowls and garnish each serving with a sprinkle of chopped celery leaves.