Preheat your oven to 400°F (200°C) and grease an 8-inch square baking dish with butter.

Peel and slice russet potatoes into ¼-inch thick rounds and place them in a bowl of cold water.

Peel and slice the yellow onion thinly.

Layer half of the potato slices and half of the onion slices in the prepared dish, then repeat the process.

In a saucepan, melt butter over medium-low heat, whisk in flour and salt until it turns golden.

Gradually add warmed milk and stir until the sauce thickens; then remove from heat and stir in cheddar cheese until melted.

Pour the cheese sauce over the layered potatoes and onions evenly.

Cover with aluminum foil and bake for 90 minutes until potatoes are tender and top is golden brown.

Broil for an additional 5-10 minutes for a crispy top, watching closely to prevent burning.