Chop the tough outer green leaves and bottom bulb from the leeks and discard. Clean the leeks well and chop.
Peel the celeriac and cut into 1 inch cubes.
In a medium sized pot, heat the butter on medium-low and add the leeks and garlic and cook for 5 minutes. Then, add the celeriac, stock, salt and pepper. Bring to a boil, then put heat on low, cover, and simmer for about 25 to 30 minutes until the celeriac is tender.
Remove from heat. Add the crème fraîche, then use a hand blender to purée.
Garnish each bowl with bacon.