Preheat oven to 375 degrees F. Spray a 9″ square or 8 x 11″ baking dish very well with cooking spray.
In a medium bowl, stir together cheese, chopped green onions, sour cream, milk, garlic powder, salt and pepper to taste.
Arrange half the potato slices over the bottom of the baking dish and half the diced chicken over the potatoes. Sprinkle lightly with salt and pepper if desired. (I like a lot of salt on my potatoes.) Spread half the cheese mixture over the chicken and potatoes. Repeat these layers with the remaining ingredients.
Spray a piece of aluminum foil with cooking spray, and cover the baking dish with the sprayed side down toward the food. Bake covered for 60 minutes.
Remove foil, and increase heat to 400 degrees F. Bake for an additional 10-15 minutes or until top is bubbly and starting to brown. Serve garnished with additional sliced green onions if desired.