Heat butter in a large skillet over medium-high heat. Add the leeks and sautÃ© until softened, 2 to 3 minutes. Transfer the leeks to the insert of a 5- to 7-quart slow cooker and add the potatoes and broth. Cover the slow cooker and cook on high for 3 hours or on low for 5 to 6 hours, until the potatoes are tender. Using an immersion blender, puree the soup, or cool the soup and puree it in a blender.
Reduce the heat to low and add the milk. Cover the slow cooker and cook for an additional 1 hour. Season with salt and pepper.
Serve the soup with cheese, onions, bacon and sour cream for garnish.
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