1-Getting started: Getting started with au gratin potatoes begins with preheating your oven to 350 degrees Fahrenheit, which sets the stage for even baking. Once that’s done, peel and thinly slice your 2 pounds of moderate starch potatoes to ensure they cook uniformly and absorb the sauce well. This step helps build the foundation of the dish’s creamy texture.

2-Cooking the onions and forming roux: Next, heat 3 tablespoons of butter in a large saucepan over medium-high heat and add 1/2 cup of finely chopped yellow onion, cooking until soft and translucent for about 5-6 minutes. Then, stir in 2 cloves of minced garlic and cook for another minute to release its aroma. Follow this by adding 3 tablespoons of all-purpose flour and cooking for 1-2 minutes to form a roux, which thickens the sauce.

3-Building the Sauce and Layers: Gradually whisk in 2 cups of whole milk and 1/2 cup of heavy cream, stirring continuously to avoid lumps, and bring it to a boil before simmering until slightly thickened. Add 1 1/2 cups of shredded cheddar cheese, half of the 1/3 cup grated Parmesan cheese, and 1 1/2 teaspoons of salt, stirring until the cheese melts fully for a smooth sauce. Mix in the sliced potatoes thoroughly to coat them evenly.

4-Baking: Pour the mixture into a 2-quart casserole dish, sprinkle the remaining Parmesan cheese on top, and cover with foil. Bake covered for 1 hour or until the potatoes are tender, noting that time may vary with slice thickness. Remove the foil and bake for another 20 minutes until the top is lightly browned and moisture evaporates. Let it cool for about 5 minutes before serving, and garnish with chopped parsley if desired.