Preparing the Vegetables

1. Preheat Your Oven: Start by preheating your oven to 375°F (190°C). This ensures that your gratin will cook evenly and develop that golden crust.
2. Slice the Vegetables: Using a sharp knife or a mandoline, slice your root vegetables into thin, even pieces—about 1/8 inch thick. This helps them cook evenly and absorb all that cheesy goodness.
3. Chop the Onion and Garlic: Finely chop the onion and mince the garlic. The onion adds sweetness, while the garlic brings in that wonderful aromatic flavor.

Making the Cheese Sauce

4. Sauté the Onion and Garlic: In a large skillet over medium heat, melt 1 tablespoon of butter. Add the chopped onion and sauté until translucent, about 3-4 minutes. Stir in the minced garlic and cook for another minute, being careful not to burn it.
5. Add the Cream: Pour in the heavy cream and bring it to a gentle simmer. Stir in the thyme, salt, and pepper. This step will infuse the cream with all those delicious flavors.
6. Add the Cheeses: Gradually whisk in the Gruyère, cheddar, and Parmesan cheese until melted and smooth. If the sauce seems too thick, you can add a splash more cream to reach your desired consistency.

Layering the Gratin

7. Grease the Baking Dish: Use the remaining tablespoon of butter to grease a 9×13-inch baking dish (or any similar-sized dish). This will help prevent sticking and ensure easy cleanup.
8. Layer the Vegetables: Start by arranging half of the sliced root vegetables at the bottom of the dish. Sprinkle a little salt and pepper over them for added flavor.
9. Pour in the Cheese Sauce: Drizzle half of the cheese sauce over the first layer of vegetables, ensuring they’re well coated.
10. Repeat the Layers: Add the remaining root vegetables on top, followed by the rest of the cheese sauce. If you’re using breadcrumbs, sprinkle them evenly over the top for that extra crunch.

Baking the Gratin

11. Cover and Bake: Cover the baking dish with aluminum foil (to prevent burning) and place it in the preheated oven. Bake for 30 minutes.
12. Uncover and Continue Baking: After 30 minutes, remove the foil and bake for an additional 20 minutes or until the top is golden brown and bubbling. If you want an extra crispy topping, you can broil it for 2-3 minutes at the end—just keep an eye on it to prevent burning!
13. Rest Before Serving: Once done, remove the gratin from the oven and let it sit for about 10-15 minutes before serving. This resting time allows the flavors to meld and makes serving easier.