With this recipe, and by following the easy directions below, this has a white sauce that does not separate during baking...
I found this recipe in a magazine or paper(?) and have used it for over 35 years-- I've made it with 2½ lbs. potatoes, and have also 4X'd it and used 10 lbs. Turns out the same every time, and it is so easy and economical. The basics are potatoes, butter, flour, milk-- simple, huh! (Additions are optional.)
Combine 3 tablespoons melted butter and 3 tablespoons flour in a heavy medium saucepan over medium heat. Stir constantly. After it heats and bubbles for just a little while, grab a whisk and now slowly add 2 ½ milk (while some cooks might like to 'fatten this up' by using cream here, I have found that it also works with 1% milk). Blend well with the whisk until all is very smooooooth. Now, abandon the whisk and stir constantly with a stiff kind of spatula or flat-edged non-stick kind of 'pancake turner', scraping all areas of the bottom to make sure that nothing is sticking or scorching. Bring to a boil and continue cooking until it has boiled for at least two minutes in order for the flour to 'cook'. Add 1 ½ teaspoons salt, 1/4 teaspoon black pepper, and stir. Set aside.
*This white sauce is very 'basic' and can be used with whatever other kind of food that might be baked/cooked like this. Another use-- if you add just 1 additional tablespoon of butter and 1 additional tablespoon of flour to the same amount of milk, you have a great White Milk Gravy to pour over biscuits, chicken fried steak, or green/yellow beans, etc. (Again, stirring constantly over medium heat, be sure to let the hot sauce mixture bubble/boil for a minimum of two minutes, to 'cook the flour'.)
Wash, peel and thinly slice (or grate) 2 ½ pound of potatoes. (Another option would be to use a 2 lb. bag of thawed hash browns if you need to save time.)
Combine the cooked white sauce and the grated potatoes by first putting a little bit of sauce into a sprayed/greased 3-quart casserole dish, then layering potato, sauce, potato, sauce-- and cover. Bake at 350-degrees, covered for 1 hour. Using a fork, check for doneness after an hour. At this point, I usually remove cover, turn oven down to 325-degrees and bake for another 30 minutes. (Total cooking time depends on the amount and depth of the potatoes, AND the size of the container you use-- pan, dish, roaster, etc.)
OPTIONAL: To this recipe, you could add minced onion, chopped ham, or ??? For what I made today (in these photos), I added some bits of ham, and a couple slices of a small red onion.
Some cooks might like to add a can of cream of celery soup (but, I don't like the MSG and high sodium that's in it) along with a couple of extra potatoes, snipped green onions, minced celery, and also top with cheese for the last 15 minutes of baking, etc., but I think this recipe can carry itself even without those additions. At certain times, when I've wanted an even CREAMIER scalloped potato dish, I have 1½'d the recipe for the white sauce and left the amount of potatoes the same. Experiment and have fun with this recipe. I have.
Additional note: Although I have never made it yet, another scalloped potatoes recipe that looks so good, so great (while possibly being more expensive, fattening and time consuming to make) comes from someone whose food blogging I really, really enjoy. It's Pennsylvania's Browneyed Baker and her recipe for this can be found at http://www.browneyedbaker.com/2011/04/20/scalloped-potatoes-recipe/ . She writes that this was one of her 10 most popular blogging recipes in 2011.