Preheat oven to 325 degrees F. Spray 2 quarts of baking dish (or 8x12) in nonstick cooking spray. Butter or oil can be used to grease the dish.
Place the sliced potatoes as shown in these photos in three rows. The potatoes should not be too tight. You want to leave enough space so that the potato slices are slightly slanted. Place onion slices between the rows of potatoes and on top. Put them wherever they will fit.
Next make the sauce by adding 2 tablespoons of butter to a pan and heating on medium heat. Once the butter is melted and the flour has been incorporated, gradually add the milk. This will alternate with the flour. Finally, whip up the sauce vigorously by whisking away all lumps. Allow mixture to simmer. It should be similar in consistency to cream gravy.
Reduce heat to low. Stir in the shredded cheese, garlic powder as well as salt and pepper. You can adjust the seasonings to suit your taste.
To coat the potatoes and onions with the sauce, spread it evenly. Do not worry; the sauce will thicken as you bake.
Bake the pan for 45 minutes covered with foil.
After 45 minutes of baking, remove the foil. Bake the potatoes for 30-45 additional minutes, or until they are cooked and have a golden crust. (Potatoes will be done when they can be easily pierced by a fork. Remove the baking sheet from the oven. Garnish with freshly chopped parsley. Enjoy!