1-First Step: Get the oven and potatoes ready. Preheat your oven to 350°F. Lightly grease a casserole dish so the potatoes do not stick. Peel the Yukon Gold potatoes and slice them into 1/8-inch thick rounds. A mandoline works well for even slices, which helps the potatoes cook at the same pace.

2-Second Step: Start the sauce base. Melt the butter in a pan over medium heat. Add the finely chopped yellow onion and cook until softened. This usually takes a few minutes. Stir in the minced garlic and all-purpose flour, and cook for 1 minute while stirring. This step helps remove the raw flour taste and starts building the creamy sauce.

3-Third Step: Add the milk and cream. Slowly whisk in the whole milk and heavy cream. Pour a little at a time so the sauce stays smooth. Bring the mixture to a boil, then reduce the heat and simmer for about 5 minutes until it thickens. Stir often so the sauce does not scorch on the bottom.

4-Fourth Step: Add the cheese and seasoning. Stir in the grated cheddar cheese, half of the Parmesan cheese, the salt, and the pepper if you are using it. Keep stirring until the cheese melts into the sauce. The mixture should look creamy, rich, and glossy. If you want to try a different flavor, this is the stage where Gruyère can replace cheddar for a nutty taste.

5-Fifth Step: Coat the potatoes. Gently fold the sliced potatoes into the cheese sauce. Make sure each slice gets coated well. This is what gives Au Gratin Potatoes their soft, rich texture in every bite.

6-Sixth Step: Assemble and bake. Transfer the coated potatoes into the greased casserole dish. Spread them out evenly, then sprinkle the remaining Parmesan over the top. Cover the dish with foil and bake for 1 hour, or until the potatoes are tender when pierced with a knife. Covering the dish keeps moisture in and helps the potatoes cook through without drying out.

7-Final Step: Brown, rest, and serve. Uncover the dish and bake for 20 more minutes until the top turns golden brown and bubbly. Let the casserole rest for 5 minutes before serving. Finish with chopped parsley for a fresh pop of color. This short rest helps the sauce settle so the portions hold together better when served.