In a large pot, melt the butter over medium heat. Add the leeks and cook until golden, about 5 minutes.
To the pot, add the turnips and thyme then continue to cook for 5 more minutes. Pour in the broth and wine (optional), and bring to a boil. Reduce to a simmer and cook until turnips are softened, about 15 minutes. Remove from heat and set aside.
Transfer the soup to a blender (or use an immersion blender in the pot) and blend until smooth. Pour back into the pot to reheat before serving. Taste test and add salt & pepper, as desired. Serve warm garnished with fresh thyme and enjoy!