1. Preheat the Oven:

• Preheat your oven to 350°F (175°C).
• Grease a 9-inch deep baking dish and set it aside.

2. Prepare the Béchamel Sauce:

• In a medium saucepan, melt the butter over medium heat.
• Add the finely chopped onion to the pan and sauté until it becomes soft and translucent. This should take around 6 minutes.
• Stir in the flour, salt, Italian seasoning, and black pepper. Continue to cook, stirring frequently, until the mixture takes on a pale golden color. This will take about 5 minutes.
• Gradually whisk in 2 cups of milk, ensuring no lumps form. Once smooth, pour in the remaining 2 cups of milk, continuing to whisk.
• Increase the heat slightly, stirring constantly, and bring the mixture to a boil.
• Once boiling, reduce the heat to low. Simmer the sauce, stirring occasionally, until it thickens and becomes creamy. This should take between 8-10 minutes.
• After tasting and adjusting for salt and pepper if necessary, remove the pan from heat. Stir in 8.8 ounces of the shredded mozzarella cheese until it’s fully melted into the sauce.

3. Assemble the Gratin:

• In a large mixing bowl, combine the thinly sliced potatoes with the prepared béchamel sauce. Make sure each potato slice is well-coated.
• Transfer this potato and sauce mixture to the greased baking dish, spreading it out evenly.
• Sprinkle the top with the reserved 1 cup of shredded mozzarella cheese.

4. Bake and Serve:

• Place the baking dish in the preheated oven.
• Bake for approximately 1 hour to 1 hour and 10 minutes, or until the potatoes are soft when pierced with a fork and the top is golden brown.
• Once done, remove from the oven and allow it to cool slightly before serving.