In large 4 or 5-quart pot, heat the olive oil over medium heat.
Add onion, leek and garlic, and sauté until the onion is translucent, about 5 minutes.
Add the chopped potatoes, celery root and bay leaf.
Stir the vegetables, and then add the stock.
Bring the soup to a boil, cover the pan, reduce heat to low and simmer 20 minutes, or until vegetables are fork tender.
Add the thyme, sea salt and pepper to taste.
Cook an additional 5 minutes.
Remove bay leaf, and puree soup with a vertical blender.
Ladle the soup into bowls and garnish with fresh herbs.