Wash potatoes and peel the potatoes. Slice the potatoes 1/4 inch thick. You can slice the potatoes with a knife, a mandolin, or even a food processor. Slice the onions the same thickness. Place the potatoes and onions into a pot. Add enough water to cover the potatoes. Add 1 teaspoon of salt to the potatoes. Cook potatoes over medium-high heat for about 10 minutes. The potatoes are done when they are fork-tender.
Prepare the sauce by adding the butter and flour to a medium-sized pot over medium heat. Cook the flour and the butter together for about a minute. The roux is done when it begins to smell like pie crust. Add 1/3 of the milk to the roux and stir until the roux becomes thick. Add in the next 1/3 of milk and stir until the sauce becomes thick. Repeat with the last remaining milk. Season sauce with 1/2 teaspoon of salt and 1/2 teaspoon of black pepper. When potatoes are done, drain and place into a small baking dish. Pour sauce over potatoes.
If desired add shredded cheese to potatoes and bake for approximately 25 minutes or until the top begins to brown.