How to Make It
Peel and thinly slice potatoes using a Mandoline slicer. Layer potatoes in a 6-quart slow cooker.
Melt butter in a small saucepan over medium-high. Add flour; cook, whisking constantly until smooth, about 1 minute. Whisk in milk. Bring to a boil, reduce heat to medium, and simmer, whisking constantly until slightly thickened, 3 to 4 minutes. Remove from heat. Whisk in salt, pepper, nutmeg, and 1 cup of the cheese.
Pour cheese sauce evenly over potatoes. Cover, and cook on HIGH until very tender, about 4 hours. Uncover; cook on high 30 minutes (to let water evaporate). Sprinkle with remaining cheese, and cook on high until melted, about 15 to 20 minutes. Sprinkle with chives.