Step 1

Place the potatoes, leeks, broth, butter, salt, thyme, and pepper in a 3- to 4-quart slow cooker and stir to combine. Cover and cook on the LOW setting until potatoes are very tender, about 8 hours.

Step 2

Blend the soup with an immersion blender until smooth. (Alternatively, transfer the vegetables, in batches, to a stand blender. Blend on low speed until smooth. Return the blended soup to the slow cooker.) Add the sour cream and whisk to combine. Ladle into bowls and serve with an extra swirl of sour cream and a sprinkle of chopped fresh thyme.