Prepare the Casserole Dish: Generously butter an 8 x 8-inch casserole dish. Spread the Simply Potatoes evenly within the dish.
Make the Creamy Sauce: In a medium saucepan, combine the milk, whipping cream, kosher salt, dry mustard, black pepper, and nutmeg. Place over medium-high heat, whisking and stirring until the mixture is nearly boiling.
Add Cheese: Reduce the heat to low, add the shredded cheddar cheese, and stir until the cheese melts completely into the sauce.
Combine with Potatoes: Pour the creamy cheese mixture evenly over the potatoes in the casserole dish. Allow the mixture to cool slightly.
Top with Butter and Paprika: Dot the top with the butter slices and sprinkle a light layer of paprika over the dish. Cover the dish tightly with plastic wrap.
Refrigerate: Place the casserole in the refrigerator to chill for at least 12 hours or overnight for best results.
Bake the Casserole: Preheat your oven to 350°F (175°C). Remove the plastic wrap from the casserole, then bake uncovered for 45 minutes, or until the top is bubbly and golden brown.
