Preheat oven to 375 °F (190 °C). Butter a 9×13-inch baking dish.
In a large skillet, melt 2 tablespoons butter over medium heat. Add 2 sliced leeks and cook 5 to 6 minutes until soft. Add 2 minced garlic cloves and cook another 30 seconds.
Sprinkle in 2 tablespoons flour and whisk for 1 minute. Slowly pour in 2 cups heavy cream, whisking until smooth. Add ½ teaspoon salt, ¼ teaspoon pepper, and ¼ teaspoon nutmeg. Simmer 2 to 3 minutes until thickened.
Layer half the 3 pounds sliced potatoes in the prepared dish. Pour over half the leek cream mixture and sprinkle with half the 1½ cups Gruyère. Repeat with remaining potatoes, sauce, and cheese. Finish with ½ cup Parmesan on top.
Cover with foil and bake 40 minutes. Uncover and bake another 20 minutes until golden and bubbly.
Let sit 10 minutes before serving so the layers set nicely.