Follow these simple steps to create your delicious Cheesy Root Veggie Gratin:

1. Preheat Oven: Preheat your oven to 375°F (190°C) and grease a 9×13-inch baking dish with butter.
2. Prepare Vegetables: Slice the potatoes, carrots, and parsnips evenly, ensuring even cooking.
3. Mix Cream and Cheese: In a bowl, combine the heavy cream, minced garlic (if using), fresh thyme, salt, and pepper.
4. Layer Vegetables: Start layering by placing half of the sliced potatoes in the prepared baking dish. Season lightly with salt and pepper.
5. Add Carrots and Parsnips: Layer half of the carrots on top of the potatoes, followed by half of the sliced parsnips.
6. Add Cheese: Sprinkle half of the grated Gruyère cheese evenly over the vegetables.
7. Repeat Layers: Repeat the layering process with the remaining potatoes, carrots, parsnips, and Gruyère cheese.
8. Pour Cream Mixture: Pour the cream and cheese mixture over the veggie layers, pressing down slightly to ensure all vegetables are coated.
9. Top with Mozzarella and Parmesan: Sprinkle the shredded mozzarella and grated Parmesan cheese on top.
10. Bake: Cover the baking dish with foil and bake for 30 minutes. Remove the foil and bake for an additional 15-20 minutes until the top is golden brown and bubbly.
11. Rest: Allow the gratin to rest for about 10 minutes before serving. This helps the dish set for easier slicing.

With these straightforward instructions, you’re well on your way to delivering a stunning Cheesy Root Veggie Gratin!