Gather all ingredients before starting the recipe.
Sauce
Melt 1 tbsp unsalted butter in a medium sauce pan over medium heat, 1 tbsp all-purpose flour and stir to combine. Cook for 1-2 minutes, then add 1 1/2 cups milk, a little a time. Whisk with a wire whisk until all the milk has been added and the sauce is smooth. Bring to a simmer, and using a wooden spoon, stir until the sauce has thickened.
Potatoes
Pre heat oven to 350 °F
Thinly slice 2 medium or one large russet potatoes and 1/3 medium sweet onion.
Butter a 4" x 8" casserole dish and place a layer of sliced potatoes on the bottom.
Season with kosher salt to taste and add a layer of thinly sliced onions.
Pour sauce over the top of the first layer, repeat this sequence until all the potatoes, onions and sauce has been used, ending with the sauce on top.
Cover dish with aluminum foil and bake potatoes in a pre heated 350ºF for about an hour. Remove foil after an hour and bake for another 15 minutes or until the top becomes golden and bubbly.