- In a large saucepan or stock pot, heat butter over medium-low heat. Add leeks and onions and saute until translucent, approximately 10-15 minutes.
- At the very end of the 10-15 minutes, add the garlic and saute until fragrant, approximately 30 seconds.
- Add the cubed potatoes to the pot, season with salt and pepper, and cover with broth.
- Add thyme sprigs and bay leaf and bring to a boil. Reduce to a simmer and let cook covered for 25- 30 minutes or until potatoes are tender when poked with a fork.
- Using an immersion blender, puree soup until desired consistency is reached. Typically vichyssoise has a fairly smooth consistency. If using a counter top blender, let cool before blending or blend in small batches, making sure to release the steam as you go. Otherwise, you'll have soup all over your counter.
- Vichyssoise is served either warm or cold, but typically cold in the summer.
- To serve, place a dollop of Greek yogurt on top, season with salt and pepper, and drizzle a bit of white truffle oil around the surface of the soup for taste.