1-First Step: Prepare the potatoes and oven. Peel the 2 pounds of Yukon Gold potatoes and slice them into 1/8-inch thick rounds. If you have a mandoline, it can help you get even slices so the potatoes cook at the same speed. Set the slices aside in a bowl of cool water if you need a few minutes before assembling, then drain and pat them dry.

2-Second Step: Start the sauce base. In a large pan over medium heat, melt the 3 tablespoons butter. Add the 1/2 cup finely chopped yellow onion and cook it until soft, about 3 to 4 minutes. Stir in the 2 cloves minced garlic and cook for about 30 seconds, just until fragrant. This quick step builds the savory flavor that makes Au Gratin Potatoes taste so good.

3-Third Step: Make the creamy cheese sauce. Sprinkle in the 3 tablespoons all-purpose flour and stir until the onions are coated. Cook for about 1 minute to remove the raw flour taste. Slowly pour in the 2 cups whole milk and 1/2 cup heavy cream, whisking as you go so the sauce stays smooth. Bring the mixture to a gentle boil, then lower the heat and simmer until it thickens. This usually takes a few minutes, and you want the sauce to coat the back of a spoon. Stir in the 1 1/2 cups shredded cheddar cheese, most of the 1/3 cup grated Parmesan cheese, the 1 1/2 teaspoons salt, and the 1/4 teaspoon ground black or white pepper until the cheese melts into a creamy sauce.

4-Fourth Step: Coat the potatoes. Add the sliced potatoes to the sauce and gently stir until every slice is coated. Be careful not to break the potatoes apart. This recipe works best when the sauce gets between the layers, so mix slowly and evenly.

5-Fifth Step: Assemble the casserole. Pour the potato mixture into the prepared 2-quart casserole dish. Spread it out evenly, then sprinkle the remaining Parmesan over the top. That final layer helps form a golden, lightly crisp finish while the potatoes bake.

6-Sixth Step: Bake until tender. Cover the dish tightly with foil and bake for 1 hour, or until the potatoes are tender when pierced with a knife. If your potato slices are slightly thicker, you may need a little extra time. Thin slices usually cook more evenly and give the best texture.

7-Seventh Step: Brown the top. Remove the foil and bake for 20 more minutes until the top turns golden and bubbly. This is the step that gives Au Gratin Potatoes their classic finish. The surface should look lightly browned, and the sauce should be thick around the edges.

8-Final Step: Rest and garnish. Let the dish sit for 5 minutes before serving. This short rest helps the sauce settle so the potatoes hold together better when scooped. Finish with chopped parsley for a fresh pop of color.