Making Cheesy Root Vegetable Gratin is simple if you follow these steps:

1. Preheat the Oven: Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with butter or cooking spray.
2. Prepare Vegetables: Wash and thinly slice the potatoes, carrots, parsnips, and turnips. Aim for uniform slices for even cooking.
3. Mix Cream and Seasoning: In a bowl, combine the heavy cream, minced garlic, fresh thyme, salt, and black pepper. Whisk until well combined.
4. Layer the Vegetables: Start by layering half of the sliced potatoes at the bottom of the greased baking dish. Follow with half of the carrots, parsnips, and turnips.
5. Add Cheese: Sprinkle a layer of Gruyère and cheddar cheese over the vegetables. Repeat the layering process with the remaining vegetables and cheese.
6. Pour Cream Mixture: Pour the cream mixture evenly over the layered vegetables and cheese. Make sure it covers everything well.
7. Top with Parmesan: Sprinkle the grated Parmesan cheese on top of the gratin for a golden, crispy finish.
8. Drizzle Olive Oil: Drizzle olive oil over the top to help it brown nicely during baking.
9. Cover and Bake: Cover the baking dish with aluminum foil and bake in the preheated oven for 30 minutes.
10. Uncover and Finish Baking: Remove the foil and bake for an additional 15-20 minutes, or until the top is golden and bubbly.
11. Cool and Serve: Once done, allow the gratin to cool for a few minutes before serving.

By following these instructions, you’ll create a delicious Cheesy Root Vegetable Gratin that will be the star of your meal.