1-First Step: Prep the potatoes and oven Start by preheating your oven to 350°F. Lightly grease a 2-quart baking dish or a similar medium casserole dish. Peel the potatoes and slice them into 1/8-inch thick rounds. Try to keep the slices even so they cook at the same speed. If needed, use a mandoline for clean, uniform slices. Set the potatoes aside in a bowl of cold water if you need to pause, then drain and pat them dry before layering.

2-Second Step: Make the sauce base Place a medium saucepan over medium heat and melt the butter. Once it begins to foam, add the chopped onion and cook for 3 to 4 minutes, stirring often, until soft and fragrant. Add the minced garlic and cook for about 30 seconds, just until you smell it. Sprinkle in the flour and stir constantly for 1 to 2 minutes. This cooks out the raw flour taste and gives the sauce body.

3-Third Step: Add the milk, cream, and seasonings Slowly whisk in the milk and heavy cream. Add the salt and black or white pepper, then keep whisking until the mixture is smooth. Bring it to a gentle simmer and let it thicken for 3 to 5 minutes. The sauce should coat the back of a spoon. Stir in 1 cup of the cheddar cheese and 1 tablespoon of the Parmesan cheese until melted. Remove the pan from the heat once the sauce is smooth and creamy.

4-Fourth Step: Layer the potatoes and sauce Arrange one-third of the potato slices in the bottom of the baking dish, slightly overlapping them. Spoon some of the cheese sauce over the potatoes, then repeat with two more layers. Try to distribute the sauce evenly so every slice gets coated. Finish by pouring any remaining sauce over the top. Sprinkle the rest of the cheddar cheese and the remaining Parmesan cheese over the surface. This gives you a bubbly top with a nice golden finish.

5-Fifth Step: Bake until tender Cover the dish with foil and bake for 50 minutes. The foil keeps the moisture in so the potatoes can soften fully. After that, remove the foil and bake for another 25 to 35 minutes. The top should be browned, the edges should bubble, and a knife inserted into the potatoes should slide in easily. If the top browns too fast, loosely cover it again for the last part of baking.

6-Final Step: Rest, garnish, and serve Let the dish rest for 10 to 15 minutes before serving. This helps the sauce settle and makes the portions easier to cut. Sprinkle chopped parsley over the top for a fresh look and a little color. Serve warm as a side with roast chicken, ham, steak, or a simple green salad. If you like potato dishes with a creamy finish, this recipe belongs in your regular rotation.