In a large skillet, melt butter over medium. Add leeks and cook about 8 minutes, until softened but not browned.Advertisement
Transfer to a 5-quart slow cooker. Stir in potatoes and next 4 ingredients. Cook on HIGH for 4 hours or LOW for 5 hours. Discard thyme stems and bay leaves. Let cool slightly.
Using an immersion blender, puree until smooth. Sprinkle with chives and pepper, and serve with kale salad on the side.