How to Make It
Spray the insert of a 6-quart slow cooker with cooking spray. Arrange the celery root and potatoes in the insert in alternating layers, seasoning lightly with salt and pepper between each layer.
In a medium pot, melt the butter over medium. Add the flour and cook, whisking constantly, until the mixture is light golden brown, with a nutty fragrance and the texture of oatmeal, about 4 minutes. Slowly whisk in the milk. Whisk constantly until the mixture comes to a rapid simmer and thickens, about 5 minutes. Whisk in all but 1 cup of the grated cheese until smooth. Add the nutmeg and season with salt and pepper.
Ladle the milk mixture over the layered vegetables and shake the vessel lightly to saturate the layers and remove any air pockets. Line the top of the slow cooker with a layer of paper towels and cover with the lid. Cook on low heat until the vegetables are tender and the sauce is bubbly and thickened, about 2 to 3 hours.
Prepare the broiler. Remove the lid and paper towels from slow cooker. Sprinkle the remaining cheese over top of the gratin. Remove the ceramic insert from the slow cooker base and transfer it to the oven. Broil until cheese is browned and bubbling, about 3 to 4 minutes. Let cool 15 minutes before serving.