Put 2 tablespoons of butter in a pot over medium heat. Once the butter
has melted, add the leeks, onion, bay leaves, and thyme
and stir until the onions
are translucent. Add the white wine
and reduce the liquid by 2/3. Add the diced celeriac and potato, stir. Add the vegetable stock and bring up to a boil, then reduce the heat to a simmer
. Cook until the celeriac is tender, approximately 35 to 40 minutes.
Carefully transfer the mixture into a food processor
and puree until smooth. Stir in the cream and remaining tablespoon of butter. Season the soup
with salt, pepper, and nutmeg
To serve, garnish the soup with sour cream
and black sesame seeds.