Generously grease a 9x13 pan with butter. Peel all of the potatoes. Preheat the oven to 375 degrees F.
Stir the salt, pepper and onion powder in a tiny bowl until combined. Divide it into three sections.
You will be making three layers of potatoes in the pan. Start by slicing 1 pound of potatoes (about 3 medium-to-large red potatoes) thinly (⅛-inch thick). A mandoline works well for this, but you could use a box grater or a food processor.
Blot the slices with paper towels to dry them and arrange them evenly in the pan, overlapping slightly and stacking as needed. (Continue blotting the potatoes with a paper towel as needed. You don't want soggy potatoes.)
Sprinkle on ⅓ of the total salt, pepper and onion powder mixture. Sprinkle 1 ½ tablespoons of flour over the potatoes. Cut up 2 tablespoons of butter into squares and dot them over the potatoes.
Slice another pound of potatoes as before and blot them dry. Arrange them in a new layer in the pan, stacking as needed. Sprinkle on half of the remaining salt, pepper, onion powder. Sprinkle on 1 ½ tablespoons of flour, reserving an even tablespoon of flour for the top layer. Cut up 2 tablespoons of butter and dot the butter over the potatoes.
Slice your final pound of potatoes as previously and blot them dry. Arrange the potatoes in an even layer, stacking as needed. Sprinkle on the remaining salt, pepper, onion powder. Evenly sprinkle on 1 tablespoon of flour (avoid having clumps of flour on the top layer, or you'll have raw flour on top after baking.) Cut up 2 tablespoons of butter and dot the pieces over the potatoes.
Pour the milk over the potatoes to fill the bottom of the pan and barely cover the potatoes.
Bake uncovered for 70-80 minutes. The potatoes are done when the entire surface is golden brown and crispy. The edges of the pan will have dark brown crust. (Watch the potatoes carefully starting at the 65-minute mark, since oven times vary.)
Refrigerate leftovers for up to five days. Reheat leftovers in the microwave.
Mom bakes her potatoes at 350 degrees for two hours, but I sped up the process by baking them at 375 degrees.
If using a larger pan such as a 10x15, use 12 large, red potatoes, a stick of butter (8 tablespoons) and slightly more flour.
Recipe adapted from Betty Crocker.
Best potatoes to use here
We've always used red potatoes, and they work really well here. If I had to pick a substitute for the red potatoes, I'd go with Yukon Gold. I'm guessing russets, which are on the dry side, might be too mealy here.