Prepare the Ingredients

1. Preheat Oven: Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with butter.
2. Sauté the Leeks: In a large skillet, melt 1 tablespoon of butter over medium heat. Add the sliced leeks and garlic, cooking until softened, about 5 minutes. Season with salt and pepper, then set aside.

Assemble the Gratin

1. Layer the Potatoes: Start by layering half of the sliced potatoes in the prepared baking dish, overlapping them slightly.
2. Add Leeks and Cheese: Spread the sautéed leeks evenly over the potatoes, then sprinkle with half of the Gruyère cheese.
3. Repeat Layers: Add another layer of the remaining potatoes, followed by the rest of the leeks and Gruyère cheese.
4. Pour the Cream: In a separate bowl, mix the heavy cream with salt and pepper. Pour the cream evenly over the layered potatoes.

Bake the Gratin

1. Add Parmesan: Sprinkle the grated Parmesan cheese over the top of the gratin, along with the remaining tablespoon of butter cut into small pieces.
2. Cover and Bake: Cover the baking dish with aluminum foil and bake for 30 minutes.
3. Uncover and Brown: Remove the foil and bake for an additional 20-25 minutes, or until the top is golden brown and bubbly.