Preheat the oven to 350 degrees. Lightly grease a 9x13 inch baking dish with cooking spray. Set aside.
Wash and dry the potatoes. Thinly cut the potatoes into 1/8-inch slices. Next, thinly slice the onions. Attractively arrange the sliced potatoes and onions in the prepared baking dish. Lay stacks of potatoes and onions upright on their sides until the baking dish is full.
In a large saucepan, melt the butter over low heat. Add flour, salt, and pepper. Stir until smooth. Gradually add the heavy cream to the flour mixture. Bring to a boil and cook, frequently stirring for about 2 minutes or until the sauce has thickened.
Remove the sauce from the heat and stir in both cheddar and Gruyere cheeses. Stir until melted. Next, pour the sauce over the potatoes and onions in the baking dish.
Cover the baking dish with tin foil — bake for 1 hour. After an hour, remove the tin foil and continue baking for an additional 30 minutes or until the potatoes are fork-tender and golden brown. Serve immediately. Enjoy!
When making au gratin potatoes, the all-purpose Yukon gold is a great choice. The Yukon gold does well to hold its shape, during baking. Learn more about the many types of potatoes.