Creating Cheesy Au Gratin Potatoes is easy if you follow these simple steps:

1. Preheat the Oven: Preheat your oven to 375°F (190°C).
2. Prepare the Baking Dish: Grease a 9×13-inch baking dish with a little butter.
3. Slice the Potatoes: Peel and thinly slice the potatoes to ensure even cooking.
4. Sauté Onions and Garlic: In a medium skillet, melt the butter over medium heat. Add the chopped onion and garlic, sautéing until softened, about 5 minutes.
5. Create the Cheese Sauce: In a large bowl, combine heavy cream, salt, black pepper, and cayenne pepper. Stir in the cheddar, Gruyère, and half of the Parmesan cheese.
6. Layer the Potatoes: Place half of the potato slices at the bottom of the greased baking dish. Sprinkle with half the sautéed onions and garlic.
7. Add the Cheese Sauce: Pour half of the cheese sauce over the first layer and repeat the layers with the remaining potatoes, onions, and cheese sauce.
8. Top with Parmesan: Sprinkle the remaining Parmesan cheese evenly over the top layer.
9. Bake: Cover the dish with aluminum foil and bake for 30 minutes. Remove the foil and bake for an additional 15-20 minutes, or until the top is golden brown and the potatoes are tender.
10. Cool and Garnish: Once out of the oven, let it cool for 5-10 minutes. Garnish with fresh chives, if desired.

These steps will guide you in making an incredible batch of Cheesy Au Gratin Potatoes that will steal the show at any meal.