Heat the butter in a large soup pot over medium-low heat until it just begins to melt and bubble slightly. Add the onion, leeks, and a pinch of salt. Cover and sauté over medium-low heat for 8 minutes, stirring once, until soft and translucent. Add garlic and thyme, and sauté, uncovered, for another 2 minutes.

Add butternut squash and sauté for 3-5 minutes. Add chicken broth and bring to a boil. Reduce heat to low and simmer, uncovered, for 20-30 minutes.

Place an immersion blender in the pan and carefully blend the soup until very smooth (alternatively, you can carefully transfer the soup to a blender and puree until smooth). Once the soup is the right consistency, add the champagne vinegar (this brightens the flavor) and bring back to a simmer for another 10-15 minutes, or until it has thickened slightly. Season with salt and pepper. At this point, you can let the soup cool slightly, then refrigerate for up to 8 hours until ready to reheat and serve.

Spoon the soup into serving bowls with 1-2 teaspoons of crème fraiche on top and sprinkle with fresh chives.