Preheat oven to 400°F.
Spray a quarter-sized, or 9x13-inch, casserole dish (or a preheated cast iron skillet) with non-stick cooking spray.
Wash potatoes and cut off the ends. Using a mandolin slicer or food processor, thinly slice the potatoes and set aside.
In a medium saucepan over medium heat, mix soup, milk, onions and Greek yogurt. Stir frequently. Once bubbling, fold in 1 cup of the Cooper Cheese, reserving 1/2 cup for later. Stir until the mixture has melted into a nice cheese sauce.
Layer half of the potatoes in the baking dish or skillet. Season with salt and pepper to taste. Pour half the cheese sauce over the potatoes. If you choose to add optional toppings, such as bacon or ham pieces, sprinkle in half of your prepared toppings. Layer the remaining potatoes into the dish or skillet. Salt and pepper again to taste. Pour remaining cheese sauce over the potatoes and sprinkle on any remaining optional toppings.
Place casserole in the oven and bake for 1 hour, or until the potatoes are cooked through.
Sprinkle the remaining 1/2 cup Cooper Cheese and fresh chives over the top of the casserole and bake another 5-10 minutes, or until the cheese is melted and the top of the au gratin potatoes is lightly browned.