Preheat the oven to 375°F. Spray a baking dish with nonstick cooking spray.
Slice the potatoes into thin slices. Slice the onion into thin strips.
Spread half of the potatoes in the casserole dish, layering as you go.Top with sliced onions.Add the last half of potato slices in a layer. Set aside for now.
Heat the butter in a pot over medium-high heat, about 1-2 minutes.
Add flour, salt and pepper and mix together until it's smooth and bubbly, about 1 minute.
Add the milk and bring it to a boil (will take 3-4 minutes) - stir often.Once boiling, stir frequently for 1 minute while it's bubbling, then remove from heat.
Add 1 ½ cups of the shredded cheese and mix together until melted, smooth and creamy.
Pour the sauce evenly overtop of the potatoes in casserole dish. Cover the dish with aluminum foil and bake for 1 hr 20 minutes.
Remove the foil after 1 hour cooking. Sprinkle ½ cup cheese and breadcrumbs overtop and bake for additional 20-30 minutes, or until golden and crispy.
Let cool 10 minutes, then dish on to plates to serve.
Recipe Notes:
Scrub the potatoes and slice off any bruises / bad spots. Then slice each potato thin, about ½ thick. There’s no need for a mandoline slicer, however these are definitely handy if you have one.
It’s okay to use potatoes with small sprouts. These can be snapped off with your fingers or sliced off with a knife.
Potatoes should be completely dry before assembling the recipe. Pat them with a paper towel. This will produce crispier potatoes.
Lightly grease the casserole dish with nonstick cooking spray before adding potatoes.
While prepping the cheese sauce, stir constantly to avoid any burning.
Feel free to add more grated cheese with breadcrumbs to make it extra cheesy.
This recipe can be made gluten-free by using a gluten-free flour and breadcrumb mixture.