1-First Step: Prep the potatoes and baking dish Start by peeling the potatoes and slicing them into one-eighth inch thick rounds. Keeping the slices even matters because it helps them cook at the same rate. A mandoline makes this faster, but a sharp knife works too if you go slowly. Grease a baking dish with butter so the potatoes do not stick while they bake. If you are cooking for a crowd, a 9 by 13 inch dish works well. For a smaller household, you can use a smaller casserole dish and slightly reduce the baking time. This recipe is easy to scale up or down depending on who is at the table.

2-Second Step: Cook the onion and garlic Melt the 3 tablespoons of butter in a saucepan over medium heat. Add the finely chopped yellow onion and cook until it turns soft and translucent, about 3 to 5 minutes. Stir in the minced garlic and cook for 30 seconds more. That short cooking time helps the garlic smell sweet instead of sharp. This step gives the sauce more flavor than simply mixing raw onion and garlic into the dish. It is a small detail, but it makes the final potatoes taste more rounded and homey.

3-Third Step: Build the creamy sauce Sprinkle in the 3 tablespoons of all-purpose flour and stir constantly for about 1 minute. This creates a light roux, which is what thickens the sauce. Slowly whisk in the 2 cups of whole milk and 1/2 cup of heavy cream, making sure the mixture stays smooth. Keep cooking over medium heat until the sauce starts to thicken. Stir often so it does not catch on the bottom of the pan. Add the 1 1/2 teaspoons salt and the optional 1/4 teaspoon freshly ground black or white pepper. Then stir in 1 cup of the grated cheddar and half of the Parmesan. The sauce should look creamy, glossy, and ready to coat the potatoes.

4-Fourth Step: Layer the potatoes and sauce Arrange about one-third of the potato slices in the bottom of the greased baking dish. Spoon some of the sauce over the top, then repeat with another layer of potatoes and more sauce. Continue until all the potatoes and sauce are used. The last layer should be well covered so the top bakes evenly and stays moist underneath. Sprinkle the remaining 1/2 cup cheddar and the rest of the Parmesan over the top. This final layer gives the potatoes that classic golden finish that makes Au Gratin Potatoes so appealing. If you like a deeper cheese flavor, you can add a little extra Parmesan on top, but do not overload the dish or it may brown too quickly.

5-Fifth Step: Bake until tender Cover the dish with foil and bake at 350°F for about 50 to 60 minutes. The foil helps trap steam so the potatoes soften without drying out. After that, remove the foil and bake for another 20 to 30 minutes until the top is golden and the potatoes are fork-tender. If the top starts browning too fast, loosely place the foil back over it. Oven temperatures can vary, so check the potatoes near the end by sliding a knife into the center. It should go in easily with little resistance.

6-Final Step: Rest, garnish, and serve Let the dish rest for about 10 minutes before serving. This helps the sauce settle and makes scooping easier. Finish with chopped parsley for a fresh color contrast and a light herbal note. Serve your Au Gratin Potatoes with roast chicken, ham, steak, or even a simple green salad. They also pair well with other comfort food favorites. If you want more side dish ideas, you may also like our favorite side dish recipes and our holiday dinner ideas for easy menu planning.