Prep: Preheat oven to 350 degrees F. Spray baking dish with cooking spray. Gather ingredients.
In a large sauce pan melt butter over medium low heat.
Chop the onion to a medium dice.
Your butter should be melted but not spattering.
Add onions to butter.
Stir to combine. Cook over medium heat until onions soften and start to turn clear.
While the onions cook peel your potatoes.
Ad slice them into thin slices. The thinner the better.
Arrange the potatoes in the sprayed baking dish.
The onions should be cooked now. Add 1/4 cup flour to the onion/butter mixture and stir well.
Allow this to cook 1-2 minutes, stirring the whole time.
Measure 2 cups of milk.
Carefully add milk to onions stirring to combine.
Raise heat to medium high and stir very frequently until boiling and thickened.
While stirring add 1 tsp salt.
Add 1/4 tsp pepper.
When the mixture boils and has thickened remove from heat.
Pour carefully over the sliced potatoes.
Shake pan to settle the sauce and sprinkle with paprika.
Cover and bake for 35 minutes. Uncover and bake an additional 35 minutes until hot and bubbly and potatoes are cooked. Enjoy!