In a large saucepan, melt the butter over medium heat. Add the leeks, onion and salt, and turn the heat down to low. sauté until vegetables are softened but not browned, stirring frequently, about 10 minutes.
Raise the heat to medium high and stir in the potatoes. Add the broth, water, bay leaf and thyme sprig and bring to a boil.
Reduce the heat to low and simmer until the potatoes are tender, about 10 minutes.
Remove from heat. Take out and discard the thyme sprig and bay leaf. Puree the soup in the pot with an immersion blender, or transfer to a regular blender and process in two batches.
Stir in the cream, taste, and add salt and pepper if necessary.
*If you would like to freeze this soup, it is best to do it before adding the cream.