I've used this recipe for many years and most people say they are the best scalloped potatoes they have had. I believe I got the recipe from Julia Child's "Way To Cook" book but have since incorporated it into my own favorite dishes.
There are several key things to know when making this dish. After peeling, use a food processor to quickly and thinly slice good quality white potatoes. The processor ensures speed and a very evenly sliced batch of potatoes. Place them in lightly salted water to keep them away from the air while you make the cream sauce. Use that same food processor, blade change required, to finely chop an onion that you will sprinkle between the layers later.
You may make it up to a day ahead, covering it tightly with plastic wrap having ensured that all the potatoes are covered with the cream sauce and refrigerating. Then just remove from the refrigerator for about 2 hours prior to baking.
As the sauce is made and thickened in advance, you need not worry about if it will thicken properly in the oven. Neither will you need to worry with stirring it during baking. Note that you should use whole milk and not cream or half and half. The higher fat content in these products will not produce the desire results, contrary to what you may be thinking. Just stick with whole milk. The potatoes will brown and come out lovely and delicious every time. If you prefer, sprinkle with a mixture of bread crumbs and grated cheese about 30 minutes before the potatoes are completed.
4 pounds potatoes, peeled and sliced
6 tablespoons unsalted butter
6 tablespoons flour
2 teaspoons salt
1/2 teaspoon white pepper
5 cups whole milk (not cream)
1 medium onion, finely chopped
2 tablespoons butter, sliced
Prepare potatoes by peeling and slicing in a food processor to ensure consistent thickness. Potatoes should be sliced using the 4mm food processor blade for traditional scalloped potatoes. Use the 2mm blade for a very thinly sliced potato and make more layers. Place sliced potatoes in salted water while proceeding with the following steps.
Change the food processor to the chopping blade and finely chop the onion.
Prepare a Rue: Heat the unsalted butter in a heavy bottom saucepan over low heat until melted. Blend in flour, salt and pepper using a wooden spoon. Cook over low heat, stirring constantly, until mixture is smooth and bubbly; remove from heat.
Whisk in milk. Move back on heat and heat to a low boil, stirring constantly. Mixture will become thick. Cook, whisking rapidly, for one minute. Remove from heat.
Generously butter a 9 inch by 12 inch glass baking dish. Drain the potatoes very well. Place a layer over potatoes on the bottom of the buttered dish. Top with a layer of the cream sauce and sprinkle with some of the onions. Repeat until all of the ingredients have been used ending with a top layer of white cream sauce. Dot with sliced butter.
Cover and bake in 350F degree convection oven 30 minutes (or 375F without convection). Uncover and cook until potatoes are tender, about 1 hour more. Let stand 15-30 minutes before serving.