Start by preheating your oven to 400°F and lightly spray an 8x8-inch casserole dish with a nonstick spray.

Next, layer the potatoes and sliced onions in the prepared dish, alternating them nicely.

In a medium saucepan, melt the butter over medium heat. Once melted, whisk in the flour and salt for about a minute, ensuring to stir constantly.

Gradually stir in the milk and continue cooking until the mixture thickens up.

Now, stir in all the shredded cheese until it's completely melted and smooth, which should take about 30-60 seconds.

Pour that delicious cheese sauce over your layered potatoes and onions, then cover the dish with aluminum foil.

Pop it in the oven and bake for 85-90 minutes, or until the potatoes feel tender when pierced.

For an extra crispy topping, switch your oven to broil once the baking time is up, and broil until you see a golden brown crust form.

Finally, season with salt and pepper to taste before serving it warm to your loved ones!