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.