1-First step: Prep the potatoes and oven Preheat the oven to 350°F. Peel 2 pounds of Yukon Gold potatoes, then slice them into 1/8-inch rounds. Try to keep the slices as even as possible so they bake at the same speed. If you are working ahead, you can place the sliced potatoes in cold water for a short time and drain them well before cooking.

2-Second step: Build the onion and garlic base Melt 3 tablespoons butter in a saucepan over medium heat. Add the 1/2 cup finely chopped yellow onion and cook until soft and fragrant, about 4 to 5 minutes. Stir in 2 minced garlic cloves and cook for 1 minute more. This step gives the sauce a deeper savory taste.

3-Third step: Make the creamy sauce Sprinkle in 3 tablespoons all-purpose flour and stir well so the butter mixture coats the flour. Cook for about 1 minute, which helps remove the raw flour taste. Slowly whisk in 2 cups whole milk and 1/2 cup heavy cream. Keep stirring as the sauce comes to a boil, then reduce the heat and simmer until it starts to thicken slightly.

4-Fourth step: Add the cheese and seasonings Stir in 1 1/2 cups shredded cheddar cheese, half of the 1/3 cup grated Parmesan cheese, 1 1/2 teaspoons salt, and 1/4 teaspoon freshly ground black or white pepper. Keep stirring until the cheese melts into the sauce and the mixture looks smooth and creamy. At this point, the sauce should smell rich and taste well seasoned.

5-Fifth step: Combine with the potatoes Add the sliced potatoes directly into the sauce and stir gently until every piece is coated. Be careful not to break the slices too much. Once coated, transfer everything to a 2-quart casserole dish and spread it into an even layer. Sprinkle the remaining Parmesan cheese over the top for that classic baked finish.

6-Sixth step: Bake until tender Cover the dish with foil and bake for 1 hour, or until the potatoes are tender when pierced with a knife. If your slices are a little thicker than 1/8 inch, they may need a few extra minutes. Once the potatoes are tender, remove the foil and bake for another 20 minutes. This final uncovered bake helps the top turn golden and slightly crisp.

7-Final step: Rest and serve Let the casserole sit for 5 minutes before serving. This short rest time helps the sauce settle and keeps the dish from being too loose. Finish with chopped parsley for a fresh look, then serve warm.