Step 1

Preheat oven to 400°F (200 degrees C).

Step 2

Butter an 8x8 casserole dish.

Step 3

Arrange the potatoes and onions in the prepared baking dish, staggering potato and then onion.

Step 4

Season with salt and pepper to taste.

Step 5

In a medium-size saucepan, melt butter over medium heat. Mix in the flour and salt, and stir constantly with a whisk for one minute. Stir in milk. Cook until mixture has thickened.

Step 6

Stir in cheese all at once, and continue stirring until melted and smooth, about 30 to 60 seconds.

Step 7

Pour cheese over the potatoes, and cover the dish with aluminum foil.

Step 8

Bake 1 1/2 hours (90min.) in the preheated oven.

Step 9

For a crisp topping, change the oven setting to broil for 5 minutes before removing the dish.