In a large saucepan, melt the butter over medium heat. Add the leeks, onion and salt, and turn the heat down to medium-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, bay leaf and thyme sprig and bring to a boil.
Reduce the heat to medium 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 pepper and cream, taste, and add additional salt and pepper to taste.
Video
Notes
Storage tipsThis soup keeps well in an airtight container in the refrigerator for 3-4 days.If you would like to freeze this soup, it is best to do it before adding the cream.