Peel and slice potatoes into 1/8-inch rounds (a mandoline makes this faster and more uniform), then thinly slice the onion into half-moons. Preheat your oven to 400°F and lightly grease a 9x13 inch baking dish or similar 3-quart casserole. Having everything prepped before you start cooking the sauce ensures smooth timing.

Arrange the sliced potatoes and onions in the greased baking dish in overlapping layers, seasoning lightly with salt and black pepper as you go. This helps distribute seasoning throughout rather than just on top. Start and end with potatoes for a neater appearance and better structure.

In a medium saucepan, melt butter over medium heat, then whisk in flour and salt and cook for about 1 minute, stirring constantly to create a smooth roux—this cooks out the raw flour taste. Gradually pour in room-temperature milk while whisking to prevent lumps, then increase heat to medium-high and stir frequently until the sauce thickens, about 5-7 minutes. I always use room-temperature milk because cold milk can cause the sauce to break or curdle. Once thickened, remove from heat and stir in freshly shredded cheddar cheese until completely melted and smooth, about 30-60 seconds.

Pour the cheese sauce from Step 3 evenly over the layered potatoes and onions, using a spatula to gently spread it and ensure the sauce reaches all layers. Cover the dish tightly with aluminum foil and bake at 400°F for 85-90 minutes, until the potatoes are tender when pierced with a fork and the sauce is bubbling at the edges.

Remove the foil carefully and place the baking dish under the broiler for 2-4 minutes, watching closely, until the top turns golden brown and crispy. For a crispier crust, I like to broil a bit longer, but keep an eye on it since broilers vary in intensity—you want golden, not burnt.

Let the scalloped potatoes rest for 5 minutes before serving—this allows the sauce to set slightly so it holds together better when plated. Taste and adjust seasoning with additional salt and pepper if needed, then serve warm.