1-First Step: Prepare the potatoes and baking dish Peel 2 pounds of Yukon Gold potatoes and slice them into 1/8-inch thick rounds. Try to keep the slices even so they cook at the same pace. A mandoline works well here, but a sharp knife is fine too if you take your time. Lightly grease a 2-quart casserole dish and set it aside.

2-Second Step: Start the sauce base Place a pan over medium heat and melt 3 tablespoons butter. Add 1/2 cup finely chopped yellow onion and 2 minced garlic cloves. Cook them until soft and fragrant, about 3 to 4 minutes. This first layer of flavor gives the sauce a sweet and savory start.

3-Third Step: Add the flour Stir in 3 tablespoons all-purpose flour and cook it briefly for about 1 minute. Keep stirring so the flour does not clump or taste raw. This step helps the sauce thicken later and gives your Au Gratin Potatoes that creamy, spoon-coating texture.

4-Fourth Step: Add milk and cream slowly Pour in 2 cups whole milk and 1/2 cup heavy cream a little at a time while stirring. This keeps the sauce smooth. Bring the mixture to a boil, then lower the heat and let it simmer until it thickens slightly. It should coat the back of a spoon but still pour easily.

5-Fifth Step: Melt in the cheese and seasonings Stir in 1 1/2 cups shredded 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. Stir until the cheese melts into a smooth sauce. If you like a stronger cheese flavor, this is the point where you can add a little extra cheddar or a handful of Gruyere.

6-Sixth Step: Fold in the potato slices Gently mix the sliced potatoes into the sauce so each piece gets coated. Take your time here so the slices stay intact. If you are making this for a gluten-free meal, the sauce will still work well as long as you use a gluten-free thickener earlier in the process.

7-Seventh Step: Assemble the casserole Transfer the potato mixture to the prepared 2-quart casserole dish. Spread it out evenly, then top with the remaining Parmesan cheese. This top layer gives the finished dish that lightly crisp, savory crust everyone loves.

8-Eighth Step: Bake covered Cover the dish with foil and bake for 1 hour. The foil helps trap steam so the potatoes become tender without drying out. During this stage, the sauce keeps working its way into the layers.

9-Ninth Step: Bake uncovered until golden Remove the foil and bake for 20 more minutes. The top should turn golden brown and the sauce should bubble around the edges. If your oven runs cool, add a few extra minutes until the center is tender.

10-Final Step: Rest and serve Take the casserole out of the oven and let it rest for 5 minutes before serving. This short rest helps the sauce settle and makes serving easier. Finish with chopped parsley for a fresh, colorful touch.