1-First Step: Get the oven and potatoes ready Preheat your oven to 350 degrees F. Peel the Yukon Gold potatoes and slice them into 1/8 inch thick rounds. Try to keep the slices even so they cook at the same rate. A mandoline can help, but a sharp knife works too if you take your time. Set the slices aside in a bowl of cool water if you need a few extra minutes before cooking.

2-Second Step: Start the sauce base Melt the 3 tablespoons butter in a saucepan over medium-high heat. Add the 1/2 cup finely chopped yellow onion and cook until softened, stirring often so it does not brown too fast. Once the onion is tender, stir in the 2 cloves minced garlic and cook for 1 more minute. The kitchen should smell wonderful at this point.

3-Third Step: Build the roux Sprinkle in the 3 tablespoons all-purpose flour and stir constantly for 1 minute. This forms a roux, which helps thicken the sauce later. Keep the mixture moving so the flour cooks but does not scorch. A light, even paste is exactly what you want here.

4-Fourth Step: Add the milk and cream slowly Gradually whisk in the 2 cups whole milk and 1/2 cup heavy cream to avoid lumps. Pour in a little at a time while whisking well after each addition. Once everything is combined, bring the sauce to a boil, then lower the heat and simmer until it thickens. This usually takes just a few minutes, but keep an eye on it so it stays smooth.

5-Fifth Step: Melt in the cheese and seasonings Stir in the 1 1/2 cups shredded cheddar cheese, half of the 1/3 cup grated Parmesan cheese, the 1 1/2 teaspoons salt, and the 1/4 teaspoon freshly ground black or white pepper. Stir until everything is fully melted and the sauce looks smooth and glossy. This is the moment when it turns into the creamy sauce that makes potatoes au gratin so good.

6-Sixth Step: Coat the potato slices Gently fold the sliced potatoes into the cheese sauce. Use a large spoon or spatula and move carefully so the slices stay intact. Every slice should get coated well, because that is what gives the finished dish its creamy, uniform texture. If you are making a larger batch, work in sections so the sauce spreads evenly.

7-Seventh Step: Assemble the casserole Transfer the potato mixture into a 2-quart casserole dish. Spread it out evenly, then top with the remaining Parmesan cheese. Cover the dish tightly with foil. This first covered bake lets the potatoes soften while keeping the sauce moist.

8-Eighth Step: Bake until tender Bake for 1 hour at 350 degrees F, or until the potatoes are tender when pierced with a fork. After that, remove the foil and bake for an additional 20 minutes. This second uncovered bake helps the top turn golden brown and slightly crisp.

9-Ninth Step: Rest and serve Let the dish rest for 5 minutes before serving. This short rest helps the sauce settle so the potatoes hold together better on the plate. Garnish with chopped parsley if desired. That little bit of green makes the dish look fresh and inviting.