03Add the wine and simmer until the liquid is reduced by half. Add the potatoes, stock, thyme and garlic. Bring to a boil, lower to a simmer and cook until potatoes are tender, about 20 minutes. Add the green part of the leeks and cook a few minutes, until just tender but still bright green.04Remove and discard the bacon and thyme sprig. Working in batches, puree the soup, adding the remaining 4 tablespoons of butter to the blender as you work. Return the soup to the pot and season to taste with salt and pepper.05Just before serving, reheat the soup and whisk in the truffle oil. Cut the truffle into slivers and stir it in. Serve with croutons if desired.