1. Melt 4 tablespoons butter in a large pot over medium heat. Add leeks, and cook until soft, about 5 minutes. Add potatoes and turnips, stirring to combine. Cover, and cook for about 10 minutes. Sprinkle flour over vegetables, stir to cover evenly, and cook for 1 minute. Add stock, season with salt and pepper to taste, and simmer until vegetables are tender, about 20 to 30 minutes.

2. With a slotted spoon, remove vegetables from pot, and place them in a food mill. Puree vegetables, and return them to pot. In a small mixing bowl, whisk together heavy cream and yolks. Temper egg mixture by adding 2 cups of soup to the bowl, stir to combine. Add egg-soup mixture to soup pot, and stir to combine. Bring to a simmer, and serve garnished with croutons and truffle shavings.