Slice potatoes evenly (I used a slicing attachment on a food processor) and cover with water in a large bowl for 30 minutes.
Meanwhile, make the sauce by melting the butter in a small pot and whisk in flour. Whisk for about 1 minute.
Slowly add half of the milk, whisking constantly. Let this come to a simmer and thicken and slowly add the second half of the milk. (Adding it in two batches helps to ensure that the flour doesn't settle to the bottom and make a clumpy sauce)
Spray a large baking casserole with cooking spray and layer in a third of the potatoes. Cover with a third of the sauce, and sprinkle with salt and pepper.
Repeat layers 2 more times, finishing with sauce, salt and pepper on top.
Bake in the preheated oven for 1 hour, covered. Remove the cover and bake another 15-30 minutes, or until tender, and starting to brown.
Recipe by Alison's Allspice at https://www.alisonsallspice.com/old-fashioned-scalloped-potatoes/