In a slow cooker, combine potatoes, leeks, onions, garlic, butter, broth, 1 tsp salt, several big grinds of black pepper and bay leaves. (Note: the ingredients may not be fully submerged in broth; this is okay, they’ll cook down considerably.) Cook on low for 4 to 6 hours.
After the soup has cooked fully, discard bay leaves. Using an immersion blender, blend the soup until smooth. Or, working in batches, transfer to a stand blender and blend on low speed until smooth. Return the puréed soup to the slow cooker. Add the cream and stir to combine. Adjust seasoning with salt and pepper, and add more broth, if needed, to thin the soup to your desired consistency. Stir in half the lemon zest and add more to taste.
Divide into bowls and top with chives.
