Preheat the Oven: Preheat your oven to 350°F (175°C).
Prepare the Potatoes: In a large bowl, combine the sliced potatoes with the melted butter, salt, and pepper. Toss to ensure the potatoes are evenly coated.
Cook the Onions and Garlic: In a frying pan over medium heat, melt a tablespoon of butter and add the chopped onion. Sauté until translucent, about 5 minutes. Add the minced garlic and cook for an additional minute, stirring frequently.
Make the Roux: In the same pan, add 3 tablespoons of flour to the onion and garlic mixture, stirring continuously to create a roux. Cook for 2 minutes, until lightly golden.
Add the Milk: Gradually pour in the 2 cups of milk, whisking constantly to avoid lumps. Continue stirring until the sauce thickens, about 5-7 minutes.
Add Cheese: Remove the sauce from heat and stir in 1 ½ cups of grated cheddar cheese until melted and smooth. Reserve the remaining cheese for topping.
Layer the Casserole: In a greased 9x13 inch baking dish, layer half of the potatoes, followed by half of the turkey. Pour half of the cheese sauce over the layering. Repeat with the remaining potatoes, turkey, and cheese sauce.
Top with Cheese: Sprinkle the reserved ½ cup of cheddar cheese over the casserole, ensuring good coverage.
Bake: Cover the dish with aluminum foil and bake in the preheated oven for 30 minutes. Remove the foil and bake for an additional 25-30 minutes, or until the potatoes are tender and the top is golden brown.
Cool and Serve: Allow the casserole to cool for a few minutes before serving. This helps the layers set and makes for easier slicing.