1. Wrap the thyme sprigs in a layer of cheesecloth, then tie with kitchen twine. In a large pot set over high heat, add 6 cups water and set over high heat. Add the leeks, potatoes and bay leaves, bring to a boil and add the salt.

2. Place the cover on the pot so the lid rests askew. Reduce the heat to medium-low and simmer for 15 minutes. Remove and discard the bay leaves and continue to simmer until the vegetables are tender, 10 to 15 minutes longer. Use tongs to remove the thyme bundle. Discard the bundle.

3. Purée the soup in batches using a food mill, blender or food processor. Taste and season with more salt if needed. Pour into an airtight container and refrigerate for at least 8 hours or overnight.

4. Stir in the milk. Divide among 8 bowls and serve with a dollop of sour cream or crème fraîche or a drizzle of heavy cream, if using.