4 pounds (1814g) round white potatoes or red-skin potatoes, peeled and rinsed
4 tablespoons (57g) butter, room temperature
1/3 cup (40g) Flourish Flour
3 1/2 cups (794g) milk, whole preferred
1 teaspoon onion powder, optional
1 teaspoon salt
Preheat the oven to 350°F. Lightly grease a 9" x 13" pan or 2-quart casserole.
To prepare the potatoes: Slice the potatoes 1/8" to 1/4" thick; if you have a Cuisinart food processor, use disk #4.
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.
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.
To make the sauce: Melt the butter in a medium-sized saucepan. Whisk in the flour, stirring until smooth.
Immediately start stirring in the milk, adding gradually and stirring constantly to prevent lumps.
When all the liquid is added, stir in the onion powder and salt.
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.
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.
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.
Remove the potatoes from the oven, and serve hot or warm.