In a large pot, heat 1 1/2 tablespoons of olive oil over medium heat. Once hot, add the diced onion and leek. Sauté for about 5 minutes, or until the onions are translucent and fragrant. Add the finely chopped garlic and diced celery, continuing to cook for another 2-3 minutes.
Stir in the cubed celeriac and potato. Season with kosher salt and white pepper to taste.
Add 4 1/2 cups of vegetable broth and toss in the bay leaf. Bring the mixture to a boil, then reduce the heat and let it simmer for about 25-30 minutes, or until the vegetables are tender.
Once the vegetables are cooked, remove the bay leaf and use an immersion blender to puree the soup until smooth. If you prefer a chunkier texture, you can blend only half of the soup and mix it back in.
For a richer flavor, stir in 1/2 cup of heavy cream at this stage. Allow the soup to heat through, then taste and adjust the seasoning with more salt and white pepper if necessary.
Ladle the Celeriac Soup into bowls and garnish with fresh chopped chives.
