Spray a soup pot with cooking spray.
Add the onion, leeks and 4 of the 5 apples. Saute over medium heat until softened, 5 to 7 minutes.
Add the chicken broth, bay leaf, and thyme.
Bring to a boil, reduce heat to low and simmer for about 15 minutes. Remove the bay leaf.
Turn off heat and set the mixture aside.
While the broth mixture is cooking, combine the evaporated milk and potatoes in a separate saucepan.
Cook over medium heat until the potatoes are tender, 15 to 20 minutes. Stir frequently.
Pour the potato mixture into the soup pot. Stir to mix evenly.
In a blender or food processor, puree the soup in batches until smooth, adding the pieces of brie cheese while pureeing.
Return the pureed batch to the soup pot and heat until warmed through.
Ladle into individual bowls and garnish with thin slices of apple. Serve immediately.