Preheat oven to 425 F. and oil a casserole dish, then cut your butter and measure out all the ingredients needed.
Using a mandolin, slice your potatoes to an ⅛-inch thick and set them aside in a bowl. If you are concerned they will turn brown while you work, simply fill your bowl with water, and the potatoes will hold their bright color. (drain well before using). But if you have everything prepared in advance, you won't need to worry about that.
Eyeballing all your ingredients in thirds, lay down a third of the potatoes, then season with salt and pepper, a few evenly spaced cubes of butter, and a spoonful of flour sprinkled over the top. Repeat two more times.
Sprinkled the garlic and onion granules over the top, and pour the milk over everything.
Bake for about 20 minutes, or until the milk bubbles. Then turn down the heat to 400 and bake for another 40 minutes. If it starts to look a little dry on top, you can place a piece of foil over the potatoes in the last 20-30 minutes of baking.
Cool slightly, cut with a knife into equal portions and scoop out a serving with a spatula, onto a plate and serve.
Video
Notes
Please note that the nutrition data given here is a ballpark figure. Exact data is not possible.