Stir in 1 cup milk, salt, paprika, pepper, and nutmeg. Bring to a low boil, stirring constantly, and reduce heat to a simmer.
Stir together remaining 1/4 cup milk and cornstarch, and add to the simmering milk. Simmer for another minute, stirring constantly, then remove from heat. Add in gruyere and stir until melted.
Layer half of the sliced potatoes into an 8 inch square or circle baking dish. Pour half of the cheese-milk sauce over the potatoes. Top with the other half of the potatoes and pour the remaining cheese sauce on top. Sprinkle with mozzarella cheese, and cover with foil.
Bake in preheated oven for 35 minutes. Remove foil and bake another 5-10 minutes until potatoes are done and cheese is bubbly. Let sit for about 10 minutes before serving. Enjoy!
Recipe from Living Well Kitchen https://blog.memeinge.com/scalloped-sweetpotato-casserole/