Preheat oven to 375 degrees F.
Sauté leeks in olive oil, salt and pepper until leeks are tender.
Butter a rectangular baking dish and layer leeks, potatoes and celery root. Sprinkle thyme over each layer.
Lightly season each layer too. End with a layer of leeks.
Pour in half a cup of white wine. Then add enough chicken stock to cover.
Drizzle 1½ tbsp. of the truffle oil over the whole mixture.
Dot with butter.
Cover the dish with tinfoil and bake for 40 minutes.
Remove cover, press down gratin with spatula and drizzle another 1½ tbsp. of truffle oil over the top.
Bake for another 20 to 30 minutes until tender.
Remove from oven and drizzle remaining truffle oil over the top and serve.