Creating your Cheesy Root Veggie Gratin can be quite simple if you follow these easy steps:

1. Preheat Oven: Start by preheating your oven to 375°F (190°C).
2. Prepare Baking Dish: Lightly grease a 2-quart baking dish with olive oil.
3. Layer Vegetables: In the prepared dish, layer the sliced potatoes, carrots, and parsnips. Season with salt, pepper, and garlic powder as you layer.
4. Add Onions: Scatter the chopped onions evenly on top of the vegetables.
5. Mix Cream and Cheese: In a bowl, combine the heavy cream with half of the Gruyère and cheddar cheeses. Stir until smooth.
6. Pour Mixture: Pour the cheesy cream mixture evenly over the layered vegetables, ensuring they are well coated.
7. Top with Cheese: Sprinkle the remaining cheese on top of the gratin for a delicious, golden crust.
8. Bake: Cover the dish with aluminum foil and bake for 30 minutes. Then, remove the foil and continue baking for an additional 10-15 minutes until the cheese is bubbly and golden.
9. Cool and Serve: Once done, allow the gratin to cool for about 5-10 minutes before serving. This helps the layers set and makes for easier slicing.

These simple steps will guide you in creating this incredible Cheesy Root Veggie Gratin effortlessly.