Recipe: Creamy Au Gratin Potatoes

Ingredients

4 medium baking potatoes (sometime listed as "russet" potatoes), sliced 0.25 inches/6 mm thick
1 onion sliced into rings
Salt and pepper to taste
3 tbsp butter
3 tbsp all-purpose flour
1/2 tsp salt
2 cups milk
1.5 cups shredded sharp cheddar cheese

Directions:

Preheat oven to 400°F/200°C (verify with oven thermometer).
While oven is heating, lightly butter an oven-proof dish that is approximately 3 quarts/3 L in capacity. The 8x8 pan (2 quarts) I used the first time was not large enough. Use something on the order of 9-10 inches (~25 cm) square.
Layer half the potatoes into the bottom of the prepared dish. Top with the onion slices and add the remaining potatoes. Season with salt and pepper to taste.
In a medium saucepan, melt butter over medium heat. Stir in the flour a tbsp at a time using a whisk, making sure that the flour is completely mixed in before adding the next tbsp (you are now forming what cooks call a roux, which is used as a thickening agent). When all the flour has been added, add the salt. Whisk for one full minute. Stir in milk. Cook until mixture has thickened, approximately five minutes. Stir in cheese in small increments. Continue stirring until melted, about 30-60 seconds.
Pour cheese mix over potatoes. Cover the dish with aluminum foil, making sure to tent it to not touch the food.
Bake for 75 minutes in the oven. Remove aluminum foil and bake an additional 15 minutes uncovered to brown the top layer.