Preheat your oven to 400°F. Prepare an 8x8-inch casserole dish by lightly spraying it with nonstick cooking spray to prevent sticking.

Layer the potatoes and onions in the prepared dish, staggering the potato slices and onion rings for an even bake and a lovely presentation.

Melt the butter in a medium-sized saucepan over medium heat. Add the flour and salt, stirring constantly with a whisk for one minute until well combined and bubbly.

Pour in the room-temperature milk, stirring continuously until the mixture thickens—this usually takes about 5 minutes.

Add the shredded cheddar cheese all at once. Keep stirring until it melts completely and the sauce is silky smooth, about 30-60 seconds.

Cover the layered potatoes and onions with your creamy cheese sauce, making sure to coat everything evenly. Then cover the dish with aluminum foil.

Bake for 85-90 minutes, or until the potatoes are tender when pierced with a fork.

Switch your oven setting to broil and broil for a few minutes until the top turns golden brown and bubbly.

Season with salt and pepper to taste before serving, and enjoy every comforting bite!