1-First step: Prep the potatoes and oven Start by heating your oven to 375°F. Grease a 9×13-inch baking dish with butter or a little oil. Peel the 2 pounds of Yukon Gold potatoes, then slice them into 1/8-inch rounds. Try to keep the slices even so the layers bake at the same speed. If you have a mandoline, use it carefully and keep the slices uniform. This helps prevent hard centers and soft edges. Pat the slices dry with a clean towel if they seem wet, since too much surface moisture can thin the sauce later.

2-Second step: Build the creamy sauce Melt the 3 tablespoons butter in a medium saucepan over medium heat. Add the 1/2 cup chopped onion and cook for about 3 to 4 minutes, until the onion looks soft and glossy. Stir in the 2 minced garlic cloves and cook for 30 seconds more. Sprinkle in the 3 tablespoons flour and whisk constantly for 1 minute. This makes a smooth base for the sauce and helps it thicken without lumps. Slowly pour in the 2 cups whole milk while whisking, then add the 1/2 cup heavy cream. Keep stirring until the sauce starts to thicken, usually 3 to 5 minutes.

3-Third step: Season and add cheese Stir in the 1 1/2 teaspoons salt and the 1/4 teaspoon pepper if you are using it. Add 1 cup of the shredded cheddar cheese and 2 tablespoons of the grated Parmesan cheese. Stir until the cheese melts into the sauce. The sauce should look creamy, slightly thick, and well seasoned. If it feels too thick, add a splash of milk. If it feels too thin, let it simmer for another minute or two. Taste it before layering, since the sauce is what seasons the entire dish.

4-Fourth step: Layer the potatoes Place a thin layer of potatoes in the baking dish, slightly overlapping them. Spoon some sauce over the top, then repeat with more potatoes and more sauce. Keep layering until you use all the potatoes and sauce. Press the layers down lightly with the back of a spoon so the sauce settles between the slices. The dish should look full but not packed too tightly. A little space helps heat move through the pan.

5-Fifth step: Add the topping and bake Sprinkle the remaining 1/2 cup cheddar cheese and the remaining Parmesan over the top. Cover the dish tightly with foil and bake for 50 minutes. Then remove the foil and bake for 25 to 35 minutes more, until the top is golden and the potatoes are fork-tender. If the top browns too quickly, loosely place foil over it near the end of baking. To test doneness, insert a knife into the center. It should slide through the potatoes with little resistance. Let the dish rest for 10 to 15 minutes before serving so the sauce can settle.

6-Final step: Garnish and serve Finish with chopped parsley for a fresh pop of color. Serve the Au Gratin Potatoes warm alongside roast beef, chicken, pork, or a vegetarian main dish. The resting time is important because it helps the sauce thicken a little more and makes serving easier.