1-Preheat the oven to 400°F and lightly spray an 8×8-inch casserole dish with nonstick cooking spray to prevent sticking.

2-Arrange the potatoes and onions alternately in the prepared baking dish, creating even layers for optimal cooking.

3-In a saucepan, melt the 3 tablespoons of butter over medium heat, then stir in the 3 tablespoons of flour and ½ teaspoon of kosher salt, whisking constantly for one minute to form a smooth base.

4-Gradually whisk in the 2 cups of whole milk and cook until the mixture thickens, creating a rich sauce that’s the heart of this recipe.

5-Stir in the 1 ½ cups of shredded cheddar cheese all at once and continue stirring until it melts and becomes smooth and creamy.

6-Pour the cheese sauce evenly over the potatoes, cover the dish with aluminum foil, and bake for 85-90 minutes or until the potatoes are tender.

7-For a crisp topping, switch the oven to broil after baking and broil until golden brown, then season with salt and pepper to taste before serving.