Melt butter in large soup pot.
Add onion, fennel, and salt and cook for approximately 5 minutes until onions soften.
Stir in garlic and thyme and continue to cook for another minute.
Stir in flour and stir scraping any bits sticking to the bottom of the pot.
Stir in broth, water, celeriac, potatoes, apple and bay leaf. Bring to boil.
Reduce heat to low and cover, letting it simmer for about 30-40 minutes.
Remove bay leaf and stir in cream.
Using an immersion blender quicly puree some of the soup, making sure to mostly leave it chunky. Alternately, ladel out 3 cups into a blender, returning it to the pot when it's smooth.
Season with salt and pepper to taste.