Creating Cheesy Root Vegetable Gratin is a straightforward process if you follow these simple steps:

1. Preheat the Oven: Set your oven to 375°F (190°C) to preheat while you prepare the vegetables.
2. Prepare the Baking Dish: Grease a 9×13-inch baking dish with butter or cooking spray to prevent sticking.
3. Slice the Vegetables: Using a sharp knife or mandoline, thinly slice the potatoes, parsnips, carrots, and onions. Ensure uniform thickness for even cooking.
4. Layer the Vegetables: Arrange half of the sliced potatoes in an even layer at the bottom of the prepared baking dish. Follow with layers of parsnips, carrots, and onions.
5. Season the Layers: Sprinkle half of the salt, pepper, and thyme over the vegetable layers, ensuring even distribution.
6. Add Cheese: Evenly sprinkle 1 cup of shredded cheese over the first layer of vegetables.
7. Repeat the Layers: Add the remaining vegetable slices on top of the cheese layer, then repeat the seasoning process with the remaining salt, pepper, and thyme.
8. Prepare the Cream Mixture: In a bowl, whisk together the heavy cream, milk, minced garlic, and nutmeg (if using) until well combined.
9. Pour Cream Mixture: Carefully pour the cream mixture over the layered vegetables, ensuring it seeps into the layers.
10. Top with Cheese and Breadcrumbs: Sprinkle the remaining cheese on top of the gratin, followed by the breadcrumbs mixed with melted butter for added crunch.
11. Bake: Place the baking dish in the preheated oven and bake for 45-50 minutes or until the top is golden brown and bubbling.
12. Cool Slightly: Remove from the oven and allow it to cool for about 10 minutes before serving. This helps the gratin set and makes it easier to slice.

Following these steps will result in a delicious Cheesy Root Vegetable Gratin that is sure to satisfy your cravings.