This recipe for creamy au gratin potatoes has been one of my husbands favorites for years; he treats it like a special occasion whenever I make it. I first developed this version during a chilly late autumn when I wanted a side that felt elegant but was simple enough for a weeknight. The result is thinly sliced russet potatoes layered with sweet onion and finished in a butter and flour based cheese sauce that bakes to a golden, bubbly top. The texture is a delicate balance of tender potato and creamy sauce with a slight chew from baked cheese at the edges.

I remember the first time I brought this to a family gathering and watched everyone go back for seconds without being prompted. The sauce is smooth and mild enough to let the potato flavor shine, yet sharp enough from white Cheddar to offer a satisfying savory bite. Use a mandoline if you have one for uniform slices and a faster assembly. Over the years I made small tweaksa touch more salt, a slightly longer simmer on the sauceand this casserole became the dish my in-laws request for holidays. Its reliably comforting and always looks special on the table.

Ingredients

• Russet potatoes: Use 4 medium russet potatoes, peeled and sliced about 1/8 to 1/4 inch thick. Russets hold their shape yet become tender and soak up the sauce. If using a mandoline choose the thicker setting to avoid overly fragile slices.
• Onion: 1 medium onion, sliced into rings. A sweet onion like Vidalia or a yellow onion adds gentle pungency without fighting the cheese. Layering the rings between spuds gives subtle onion pockets in every bite.
• Butter and flour: 3 tablespoons unsalted butter and 3 tablespoons all-purpose flour form the base of the sauce. Cook the roux briefly to remove the raw flour taste before adding milk.
• Milk: 2 cups whole milk for the creamiest results. Lower fat milks will work but the sauce will be less rich. Warm the milk slightly if you like to speed thickening and avoid lumps.
• Cheddar cheese: 1 1/2 cups freshly shredded sharp white Cheddar. Freshly shredding is key for smooth melting; pre-shredded cheese contains anti-caking agents and can result in a slightly grainy sauce.
• Seasonings: 1/2 teaspoon salt plus additional salt and freshly ground black pepper to taste. Season each layer lightly to ensure even seasoning through the casserole.
• Butter for the dish: Use a bit of extra butter to grease the 2-quart casserole to prevent sticking and create a little crispy edge where the potatoes meet the dish.

Instructions

Prepare and preheat: Preheat the oven to 400 degrees F. Grease a 2-quart casserole dish with a little butter. Peel the potatoes and slice them uniformly; a mandoline is ideal for even cooking. Slice the onion into thin rings. Arrange tools and measure ingredients so assembly moves smoothly. Layer the potatoes and onion: Place half of the potato slices in an even layer in the bottom of the prepared dish. Lightly season with salt and pepper. Arrange the onion rings over the potatoes, then finish with the remaining potato slices layered on top. Season the top layer again with a little salt and pepper to distribute flavor. Make the cheese sauce: Melt 3 tablespoons butter in a medium saucepan over medium heat. Whisk in 3 tablespoons flour and 1/2 teaspoon salt. Cook, whisking constantly, for about 1 minute to remove raw flour taste. Gradually whisk in 2 cups milk, adding small amounts and whisking until smooth to avoid lumps. Cook, whisking often, until the sauce thickens and coats the back of a spoon, about 3 to 5 minutes. Remove from heat and stir in 1 1/2 cups shredded Cheddar until melted and smooth. Assemble and bake: Pour the hot cheese sauce evenly over the layered potatoes, pressing gently so sauce seeps between slices. Cover the casserole with foil and bake at 400 degrees F for about 1 hour 20 minutes or until potatoes are tender when pierced with a knife and the sauce is bubbling. Remove foil for the last 10 minutes to allow the top to brown slightly. Rest and serve: Let the casserole rest for about 10 to 15 minutes before serving to let the sauce set and make slicing easier. Garnish with chopped chives or parsley if desired and serve warm alongside roasted meats or a bright salad.

Storage Tips

Cool leftovers to room temperature quickly but safely and transfer to a shallow, airtight container. Refrigerate within two hours of baking and use within three days. To reheat, place casserole in a 375 degrees F oven covered with foil and heat until bubbling through the center. For single portions, microwave on medium power in short intervals, stirring or flipping the portion to promote even reheating. Avoid freezing; the texture of the sauce is likely to break down during thawing and reheating.

Ingredient Substitutions

If you wish to change the profile, substitute Gruyere or Swiss for some or all of the Cheddar for a nuttier flavor and better browning. To make it gluten-free, replace the all-purpose flour with a 1 to 1 gluten-free baking blend or use cornstarch: mix 2 tablespoons cornstarch with a little cold milk and whisk into the warmed milk until thickened. For a lighter version, use 2 percent milk but note the sauce will be less rich. Add diced ham or cooked leeks for a heartier dish; fold in about 1 to 1 1/2 cups of add-ins just before pouring sauce over the potatoes.

Serving Suggestions

Serve this alongside roast pork loin, baked ham, or a simply seasoned beef tenderloin for an elegant plate. For a lighter contrast, pair with a crisp green salad dressed in a tangy vinaigrette or roasted Brussels sprouts for textural contrast. Garnish with snipped chives, a drizzle of olive oil, or a few grinds of black pepper. It also works as a vegetarian main when served with a robust winter salad or sautéed mushrooms to add savory depth.

Cultural Background

Au gratin is a French technique where ingredients are topped with a browned crust, often using cheese, breadcrumbs, or buttered breadcrumbs. In this adaptation the technique is applied to thinly sliced potatoes layered with onion and finished with a cheese-laden sauce. Historically, gratin dishes emphasized transforming humble ingredients into something celebratory by adding a rich topping and time in the oven to create a golden finish.

Seasonal Adaptations

In winter, enrich the casserole by stirring cooked, chopped root vegetables such as parsnip or rutabaga into the layers. In spring, swap yellow onion for thinly sliced leeks and fold in fresh herbs like tarragon or chives. For holiday tables, incorporate a touch of nutmeg into the sauce and finish with a blend of Gruyere and Parmesan for a more complex top layer. Lighten the dish in summer by serving smaller portions with an herb-forward salad.

Meal Prep Tips

To streamline weeknight cooking, slice potatoes and onions the night before and store them submerged briefly in cold water to prevent browning. Drain well and pat dry before assembling. You can assemble the full casserole and keep it refrigerated covered for up to 12 hours, then bake when convenient. Use shallow, ovenproof containers for faster reheating and easier portioning. Label with the bake time and temperature if youre prepping for guests or a potluck.

This creamy, comforting casserole is one of those dishes that tastes like effort and looks like celebration, even when its made for an ordinary Tuesday. Give it a try, make it your own with a favorite cheese, and enjoy the way it brings people together at the table.