1-First step: Prep the potatoes and oven Start by preheating your oven to 350°F. Peel the potatoes and slice them into 1/8-inch thick rounds so they cook evenly. A mandoline makes this faster, but a sharp knife works too. Keeping the slices thin is important because it helps the potatoes soften fully while baking and lets the sauce soak in between the layers. While you prepare the potatoes, lightly grease a 2-quart casserole dish. This size works well for the recipe and helps the potatoes bake into a nice, even layer. If you are making the dish ahead, you can stop here, cover the sliced potatoes with cold water for a short time, and continue later. Just dry them well before assembling.

2-Second step: Build the flavor base Melt 3 tablespoons butter in a pan over medium-high heat. Add 1/2 cup finely chopped yellow onion and cook until soft, which usually takes just a few minutes. Then add 2 cloves minced garlic and cook for 1 minute more. This base gives the sauce a deeper flavor than using milk alone. Stir in 3 tablespoons all-purpose flour and cook for 1 to 2 minutes. This helps remove the raw flour taste and gives the sauce a smooth, steady thickness. Keep stirring so the mixture does not brown too fast. If you want a lighter flavor, you can cook the onion just until translucent instead of deeply soft.

3-Third step: Make the cheese sauce Gradually add 2 cups whole milk and 1/2 cup heavy cream, whisking as you pour. Bring the mixture to a boil, then reduce the heat and simmer until it thickens. This should look creamy and slightly spoon-coating. Slow addition matters here because it keeps the sauce smooth instead of lumpy. Now stir in 1 1/2 cups cheddar cheese, half of the 1/3 cup Parmesan cheese, 1 1/2 teaspoons salt, and 1/4 teaspoon freshly ground black or white pepper if using. Stir until the cheese melts fully. This is the heart of the dish, and it should smell rich, savory, and comforting. If you want to use a Gruyere and cheddar mix, this is the time to add it.

4-Fourth step: Coat the potatoes Add the sliced potatoes directly into the sauce and mix gently so every piece is coated. Try not to break the slices. The sauce should cling to the potatoes and settle into the layers as you transfer everything to the casserole dish. Pour the mixture into the prepared 2-quart casserole dish and spread it out evenly. Sprinkle the remaining Parmesan on top. That final layer adds flavor and helps create the browned top people love in Au Gratin Potatoes.

5-Fifth step: Bake until tender Cover the dish tightly with foil and bake for 1 hour. This trapped steam helps the potatoes cook through. Then uncover and bake for 20 more minutes to brown the top and reduce extra moisture. If the top is not as golden as you want, you can let it bake a few minutes longer while watching closely. During this stage, the sauce thickens more, and the potatoes become soft and tender. If you are baking the dish straight from the refrigerator, add extra time because the cold temperature slows the cooking. For make-ahead meals, this step matters a lot.

6-Final step: Rest and garnish Remove the casserole from the oven and let it rest for 5 minutes before serving. This short rest helps the sauce set so it is easier to scoop. Finish with chopped parsley for a fresh look and a little color contrast against the creamy cheese.