1-First Step: Get the oven and potatoes ready Preheat the oven to 350°F. Peel the Yukon Gold potatoes and slice them into 1/8-inch rounds. If you want the slices to cook evenly, a mandoline makes the job quick, but a sharp knife works too. Set the potatoes aside while you start the sauce.

2-Second Step: Cook the onion and garlic Melt the butter in a pan over medium-high heat. Add the finely chopped yellow onion and cook until soft, which usually takes a few minutes. Stir in the minced garlic and cook for another minute. This step builds a fragrant base that gives the Au Gratin Potatoes a deep, homemade taste.

3-Third Step: Make the sauce Sprinkle in the flour and cook for 1 to 2 minutes, stirring often. This helps remove the raw flour taste and gives the sauce a smooth finish. Slowly add the whole milk and heavy cream while stirring so no clumps form. Bring the mixture to a boil, then reduce the heat and simmer until it thickens slightly. At this stage, the sauce should coat a spoon. If it feels too thin, let it simmer a little longer. If you are cooking for guests who prefer extra richness, this is also the moment to add a touch more cream, though the recipe works well as written.

4-Fourth Step: Add the cheese and seasonings Stir in the grated cheddar cheese, half of the Parmesan cheese, the salt, and the black or white pepper. Keep stirring until the cheese melts fully and the sauce looks smooth. The cheese should blend into the dairy and create a velvety texture that will coat the potatoes evenly.

5-Fifth Step: Fold in the potatoes Add the sliced potatoes to the pan and mix thoroughly so every slice gets covered with sauce. Take your time here. A good coating means better flavor in every bite and fewer dry spots after baking. If you like a little extra onion flavor, this is also the stage where finely minced shallot can work in future variations.

6-Sixth Step: Transfer to the casserole dish Pour the potato mixture into a 2-quart casserole dish. Spread it out evenly, then sprinkle the remaining Parmesan cheese over the top. Cover the dish with foil and place it in the preheated oven.

7-Seventh Step: Bake until tender Bake for 1 hour, or until the potatoes are tender when pierced with a fork. Then uncover the dish and bake for another 20 minutes until the top is lightly browned. This uncovered finish gives the Au Gratin Potatoes that classic golden top everyone loves.

8-Final Step: Rest, garnish, and serve Let the dish sit for 5 minutes before serving. This short rest helps the sauce settle and makes serving easier. Garnish with chopped parsley if desired. Then bring it to the table while it is warm and creamy.