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Preheat the oven to 350°F. Lightly grease a 9" x 13" pan or 2-quart casserole.
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To prepare the potatoes: Slice the potatoes 1/8" to 1/4" thick; if you have a Cuisinart food processor, use disk #4.
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Place the sliced potatoes in a large saucepan with cold water to cover; add 1 teaspoon salt. Bring to a boil; as soon as the potatoes come to a full boil, set your timer for 1 minute. After 1 minute, remove them from the heat.
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Drain the potatoes thoroughly, and layer in the prepared pan. Salt the layers as you go, if desired; the salt in the white sauce is the bare minimum required for flavor. If you're someone who likes a bit more salt on their food, then do sprinkle some onto the potatoes as you go.
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To make the sauce: Melt the butter in a medium-sized saucepan. Whisk in the flour, stirring until smooth.
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Immediately start stirring in the milk, adding gradually and stirring constantly to prevent lumps.
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When all the liquid is added, stir in the onion powder and salt.
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Cook over medium heat, stirring frequently, until the sauce thickens just a bit; this won't be very noticeable, more like the difference in consistency between skim milk and heavy cream. Remove from the heat.
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Pour the sauce over the potatoes, pressing them down so they're nearly covered; a few slices sticking up are OK, they'll lend the dish character.
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Bake the potatoes for 1 hour, or until they're bubbly and golden brown on top. A sharp knife inserted into the center of the potatoes should find little resistance.
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Remove the potatoes from the oven, and serve hot or warm.
