Preheat oven to 350 degrees F. Lightly grease an oven-to-table baking dish with some of the butter.
Melt the rest of the butter in a saucepan or small skillet and saute the onion until it becomes translucent. Gradually add the flour to make blond roux, then slowly add the milk and allow the mixture to thicken. Season with white pepper and just a dash of salt and remove from heat. (Be careful to avoid overdoing the salt, because there is salt in the cheese.)
Layer 1/3 of the potatoes in the baking dish, followed by 1/3 of the milk sauce, and 1/4 cup of the shredded white Cheddar. Repeat the sequence twice for a total of 3 layers. You can top with the breadcrumbs before baking, but omit if making the dish gluten-free. Bake covered for about 45 minutes to 1 hour, removing the cover for the last 10 minutes. (You can speed up the cooking time by pre-microwaving for 10 to 15 minutes before baking, or even par-boiling the potatoes.) The scalloped potatoes are done when the potatoes are tender. Let rest a few minutes before serving. Garnish with sliced scallions and bread crumbs.
All kids seem to love this dish (adults too). I keep this dish visually simple using only light colored ingredients, because kids seem to prefer it without green things (like the green parts of scallions), or black stuff (like ground pepper). However, you can jazz up the color if you like. In the episode, we topped this with bread crumbs - which you can do - but I keep the basic recipe gluten free.
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