These au gratin potatoes have a special place in my family s kitchen. My husband calls them a celebration every time I make them and guests often ask for the recipe before the casserole leaves the table. I first learned this version on a rainy afternoon when I wanted something rich and comforting to pair with roast beef. The combination of tender russet slices and a smooth cheddar sauce creates that melt in your mouth texture that feels indulgent without being fussy. Over the years I refined the technique so the sauce stays creamy and the top does not overbake.

I love how simple pantry staples transform into something luxurious. The thin potato slices absorb the savory cheese sauce while baking, creating layers of flavor and a slightly crisp edge where the top slices meet the casserole. This is a dish that rewards careful slicing and patience during baking. When I serve it at holiday dinners the room always quiets for a minute because everyone takes that first bite and remembers something wonderful. Keep reading for step by step instructions, substitution ideas, storage tips, and small tricks that help this dish come out perfect every time.

Ingredients

• Russet potatoes: Use 4 medium russets, peeled and sliced 1 4 inch or thinner. Russets hold their shape while becoming tender. A mandoline speeds the process and ensures even cooking. If you prefer waxy potatoes, choose Yukon Golds for a creamier mouthfeel.
• Onion: One medium onion sliced into rings. A sweet onion such as Vidalia or Walla Walla is lovely when in season because it mellows and blends into the sauce, but a standard yellow onion works perfectly.
• Butter and flour: Three tablespoons butter and three tablespoons all purpose flour form a roux that thickens the milk into a velvety sauce. Use unsalted butter so you can control seasoning.
• Milk: Two cups whole milk gives the richest sauce. You can substitute reduced fat milk but the texture will be slightly less luxurious.
• Cheddar cheese: One and a half cups freshly shredded sharp white Cheddar. Shred your own for smoother melting and a brighter flavor; pre shredded cheese contains anti clumping agents that can affect texture.
• Seasoning: Salt, freshly ground black pepper. I include a half teaspoon salt in the sauce plus salt to taste between layers for balanced seasoning.

Instructions

Prepare and preheat: Gather all ingredients and preheat the oven to 400 degrees F. Grease a 2 quart casserole dish with butter to prevent sticking. Peel the potatoes and, using a mandoline or sharp knife, slice them about 1 4 inch thick or slightly thinner for more tender layers. Slice the onion into even rings so they layer nicely between the potatoes. Layer the vegetables: Place half of the potato slices in an overlapping single layer on the bottom of the prepared dish. Season that layer lightly with salt and freshly ground black pepper. Scatter the onion rings evenly over the potatoes then arrange the remaining potato slices on top. Season the top layer as well. Even layering promotes uniform cooking and helps the sauce penetrate each slice. Make the roux: Melt three tablespoons butter in a medium saucepan over medium heat. Add three tablespoons all purpose flour and a half teaspoon salt. Whisk constantly for about one minute to cook off the raw flour flavor. You want a smooth paste that bubbles slightly but does not brown; this is the foundation that will thicken the sauce. Finish the sauce: Gradually add two cups milk, about a quarter cup at a time, whisking well after each addition to prevent lumps. Continue to cook, whisking constantly, until the mixture thickens, about three to five minutes. Remove from heat and stir in one and a half cups shredded Cheddar cheese until fully melted and smooth. Taste and adjust seasoning. The sauce should coat the back of a spoon and be pourable. Assemble and bake: Pour the hot cheese sauce evenly over the layered potatoes. Cover the dish tightly with foil and bake at 400 degrees F until the potatoes are tender and the sauce is bubbling, about one and a half hours. Remove the foil for the last fifteen minutes if you want a lightly browned top. Let the casserole rest for five to ten minutes before serving so the sauce sets slightly and is easier to portion.

Storage Tips

Store leftovers in a shallow airtight container in the refrigerator for up to three days. To reheat, place portions in a baking dish, cover with foil to prevent drying, and warm at 375 degrees F until bubbling, about 20 to 30 minutes depending on portion size. If the top becomes too brown during baking, cover with foil and continue cooking. Do not freeze the prepared casserole; the sauce can separate upon thawing and change the texture negatively.

Ingredient Substitutions

If you d like a richer sauce swap half the milk for light cream or use all cream for a more decadent result. For cheese swaps use Gruyere or Swiss for a nuttier profile, or mix half Cheddar and half Gruyere. To make this gluten free replace the all purpose flour with a 1 to 1 gluten free flour blend or use cornstarch as a thickener, mixing one tablespoon cornstarch with two tablespoons cold milk before adding to the warm roux.

Serving Suggestions

Serve alongside roast pork loin, beef tenderloin, or a simple roasted chicken for a classic holiday plate. A bright green salad with a citrus vinaigrette or steamed green beans adds balance to the richness. Garnish with chopped chives or flat leaf parsley for color. For a heartier meal serve with a green vegetable and a crisp white wine such as Chardonnay or a light red like Pinot Noir for contrast.

Cultural Background

Au gratin has its roots in French cuisine where gratin refers to a browned crust created by baking. Traditional variations may use cream and Gruyere; the version with sliced potatoes baked in a cheese or cream sauce became popular in homes because it is comforting and uses inexpensive ingredients. Over time regional takes have appeared across the globe, often incorporating local cheeses or additions like ham or onions to reflect local tastes.

Seasonal Adaptations

Adjust this dish through the year by changing the onions and herbs. In spring use young sweet onions and a handful of chopped tarragon. In autumn swap half the Cheddar for a sharp smoked cheese and add a pinch of ground nutmeg to the sauce for warmth. For holiday tables consider adding cooked diced ham or a scattering of sautéed mushrooms to the layers for extra savoriness.

Meal Prep Tips

For make ahead convenience assemble the layered casserole, cover tightly, and refrigerate for up to 24 hours before baking. If refrigerated, allow the dish to sit at room temperature for 20 minutes before placing in the oven to reduce cooking time variance. If you plan to bake from chilled, add 10 to 15 minutes to the baking time and check for tenderness with a knife.

Success Stories

I ve received notes from readers who served this at weddings, family reunions, and Sunday dinners. One reader doubled the recipe, added leftover diced ham, and sent back a photo of an empty casserole dish and a note that said everyone went back for seconds. Another wrote that using Vidalia onions produced a sweeter, almost caramelized character that made the dish a weeknight favorite.

Every time I make these potatoes I am reminded that simple technique and quality ingredients produce memorable food. Whether you re feeding a crowd or making a quiet family dinner, this casserole brings warmth to the table and invites conversation. Try it with different cheeses and tell me what combination becomes your family favorite.