Peel the potatoes, then use a mandolin, or sharp knife for a thin slice about ¼ inches thick.
Finely chop the onions.
Spray the bottoms and sides of an 8 x 8 baking dish with non-stick cooking spray.
Combine the flour, salt and pepper together.
Being layering the potatoes and onions in the casserole dish. Followed by the flour, salt and pepper mixture.
Top with the cubed butter.
In a small saucepan, heat the milk until just before boiling.
Pour the milk over the layers in the baking dish.
Top with the shredded cheddar cheese.
Bake covered until the potatoes are fork tender (approx 50 minutes).
Uncover and bake until the cheese bubbles and browns. (approx 15 minutes)
Broil for 2 minutes to get a nice golden brown top.
Remove the dish from the oven, and let it stand for 15 - 20 minutes or until the sauce thickens.
Notes
You can tell when scalloped potatoes are cooked completely through when yoyu can easily stick a knife in the center of the casserole and the knife easily goes through the potatoes.