Add the sliced potatoes to a bowl and cover them with water. This step helps prevent browning while you prepare the rest. Microwave for about 5 minutes to par-cook the potatoes, making them tender.
Drain the water and set the potatoes aside. Preheat your oven to 410°F (200°C). This ensures your gratin bakes evenly and gets that lovely golden crust.
Melt the butter in a skillet over medium heat. You want it to be fully melted but not browned, which can affect the flavor.
Add the flour to the melted butter, stirring to create a roux. Gradually stir in the milk, ensuring there are no lumps. Keep stirring until the mixture thickens.
Season the sauce with salt, ground black pepper, and grated Parmesan cheese. Stir well to combine, and let it simmer for a couple of minutes. The sauce should be creamy and smooth.
Now it’s time to add the potato slices to the white sauce. Stir gently, making sure all the slices are coated evenly. You want that sauce to embrace every piece!
Turn off the heat and transfer the coated potatoes to a baking dish. Pour the remaining white sauce over the top. Don't skimp on the sauce—it’s the best part!
Top the dish with mozzarella cheese. This will create that beautiful, melty top we all love.
Bake in the preheated oven for 10 to 15 minutes, or until the cheese is melted and the top is golden brown. Keep an eye on it; every oven varies!
Once done, remove the gratin from the oven and let it rest for a few minutes. This helps the layers set and makes it easier to serve.
