1-First Step: Prep the potatoes and oven. Preheat the oven to 350°F. Peel the Yukon Gold potatoes and slice them into 1/8 inch thick rounds so they cook evenly. Try to keep the slices close in size for the best texture. A mandoline makes this faster, but a sharp knife works fine too.

2-Second Step: Start the sauce. Place a pan over medium-high heat and melt the 3 tablespoons of butter. Add the finely chopped yellow onion and cook for 5 to 6 minutes, stirring now and then, until it turns soft and translucent. Add the minced garlic and cook for 1 more minute. The kitchen will smell amazing at this point.

3-Third Step: Thicken the base. Stir in the 3 tablespoons of all-purpose flour and cook for 1 to 2 minutes. This step removes the raw flour taste and helps the sauce thicken later. Keep stirring so the mixture does not scorch on the bottom of the pan.

4-Fourth Step: Add the milk and cream. Slowly pour in the 2 cups of whole milk and 1/2 cup heavy cream while stirring constantly to avoid lumps. Bring the mixture to a slow boil, then reduce the heat and let it simmer until it thickens. This should look smooth, creamy, and lightly coating the back of a spoon.

5-Fifth Step: Melt in the cheese. Stir in the shredded cheddar cheese, half of the Parmesan, the 1 1/2 teaspoons salt, and the optional 1/4 teaspoon pepper. Keep stirring until the cheese melts and the sauce turns silky. If you are using a mix of Gruyère and cheddar, this is where that nutty flavor will shine.

6-Sixth Step: Coat the potatoes. Add the sliced potatoes to the sauce and mix thoroughly so every slice gets covered. Be gentle but thorough. You want the sauce to reach between the layers, not just sit on top.

7-Seventh Step: Bake until tender. Transfer the potato mixture to a 2-quart casserole dish. Sprinkle the remaining Parmesan over the top, then cover the dish with foil. Bake for 1 hour, or until the potatoes are tender when pierced with a knife.

8-Eighth Step: Brown the top. Remove the foil and bake for 20 more minutes. This helps the top turn golden and lets extra moisture evaporate, which gives you that creamy but not watery finish. If you like a deeper color, keep it in for a few extra minutes, but watch closely.

9-Final Step: Rest and serve. Let the dish rest for 5 minutes before serving. Finish with chopped parsley for a fresh touch. That short rest helps the sauce settle so the servings hold together better on the plate.