1-First Step: Prep the potatoes and oven Start by heating your oven to 375°F. Grease a baking dish that holds about 2 to 2 1/2 quarts. Peel the potatoes, then slice them into 1/8-inch rounds so they cook evenly. If you have a mandoline, this is the moment to use it, because uniform slices make a big difference in the final texture. If you are cooking for a crowd, you can prep the potatoes a little ahead and keep them in cold water for a short time, then dry them well before layering.

2-Second Step: Make the sauce base Melt the butter in a medium saucepan over medium heat. Add the chopped onion and cook for about 3 to 4 minutes, stirring often, until it turns soft and fragrant. Stir in the minced garlic and cook for 30 seconds more. Next, whisk in the flour and keep stirring for about 1 minute. This helps cook out the raw flour taste and gives the sauce a smooth start.

3-Third Step: Build the creamy cheese sauce Slowly pour in the whole milk while whisking constantly. Add the heavy cream, salt, and optional pepper. Keep cooking and stirring until the sauce thickens enough to coat the back of a spoon, which usually takes 4 to 6 minutes. Lower the heat and stir in 1 cup of the cheddar cheese and half of the Parmesan cheese until melted. The sauce should look silky, creamy, and smooth. If it seems too thick, add a splash of milk. If you are adjusting the dish for a lighter table, this is also the point where you can reduce the cheese slightly or use lighter dairy.

4-Fourth Step: Layer the potatoes and sauce Arrange one-third of the potato slices in the baking dish, slightly overlapping them. Spoon some of the cheese sauce over the layer, then repeat with two more layers. Make sure the sauce gets between the potato slices so every bite has flavor. If you want a richer top, reserve a little extra sauce to spoon over the final layer.

5-Fifth Step: Add the finishing cheese layer Pour the remaining sauce over the top. Sprinkle the rest of the cheddar and the remaining Parmesan over the surface. This top layer is what gives Au Gratin Potatoes that bubbly, browned finish people love. For a sharper flavor, you can mix cheddar with Gruyère here, which works especially well for holiday meals.

6-Final Step: Bake until tender and golden Cover the dish with foil and bake for 45 minutes. Then uncover and bake for 20 to 30 minutes more, until the potatoes are tender and the top is golden brown with bubbling edges. Let the dish rest for 10 to 15 minutes before serving so the sauce can settle. Finish with chopped parsley for a fresh, colorful touch. Serve warm and watch how quickly it disappears.