1-First Step: Preheat and prepare your ingredients Start by heating your oven to 350°F. Peel the 2 pounds of Yukon Gold potatoes and slice them into 1/8-inch thick rounds so they cook evenly. Finely chop the 1/2 cup yellow onion, mince the 2 garlic cloves, and measure out the milk, cream, flour, cheese, salt, and pepper before you start cooking. If you like to plan ahead, this is also the best time to grease your 2-quart casserole dish. Having everything ready keeps the sauce from cooking too fast while you assemble the dish.

2-Second Step: Cook the onion and garlic Place a large pan over medium heat and melt the 3 tablespoons butter. Add the chopped onion and cook it for about 5 minutes, or until soft and fragrant. Stir in the garlic and cook for 1 minute more. This quick base adds depth to the sauce. The onion should look tender, not browned, so the flavor stays mellow and blends well with the cheese.

3-Third Step: Make the creamy sauce Sprinkle in the 3 tablespoons all-purpose flour and cook for 1 to 2 minutes, stirring often. This helps remove the raw flour taste and builds the sauce base. Slowly whisk in the 2 cups whole milk and 1/2 cup heavy cream, keeping the mixture smooth as it comes together. Bring the sauce to a boil, then lower the heat and simmer for about 3 to 5 minutes until it thickens. Stir often so the bottom does not scorch. The sauce should coat a spoon lightly when it is ready.

4-Fourth Step: Add the cheese and seasonings Stir in the 1 1/2 cups sharp cheddar cheese or Gruyère, along with about 2 to 3 tablespoons of the 1/3 cup Parmesan cheese. Add the 1 1/2 teaspoons salt and 1/4 teaspoon freshly ground black or white pepper. Keep stirring until the cheese melts fully and the sauce looks smooth and glossy. If you prefer a smoother melt, Gruyère works beautifully. If you want a stronger cheddar flavor, sharp cheddar gives the dish a bold finish. For a better understanding of cheese choices and texture, you can check this helpful Harvard guide to cheese.

5-Fifth Step: Coat the potatoes Gently fold the sliced potatoes into the sauce until every round is coated. Try not to stir too hard, since the slices can break apart. The sauce should cling to the potatoes and look rich and creamy. If the mixture seems very thick, add a small splash of milk or cream. If you are making a lighter version, you can use a little extra milk, but the texture will be less rich.

6-Sixth Step: Assemble and bake Transfer everything to the prepared 2-quart casserole dish. Spread it out evenly and sprinkle the remaining Parmesan over the top. Cover the dish tightly with foil and bake for 1 hour, or until the potatoes are tender when pierced with a knife. Then remove the foil and bake for another 20 minutes until the top is lightly browned and bubbling. The uncovered baking time helps create a golden top while keeping the center creamy.

7-Final Step: Rest and serve Let the casserole rest for 5 minutes before serving. This short rest helps the sauce settle and makes it easier to scoop neat portions. Finish with chopped parsley for color and a fresh, bright touch.