Let’s get cooking with these easy steps:

1. Preheat your oven to 375°F (190°C).
2. Grease a baking dish with butter.
3. In a medium saucepan, melt the butter over medium heat. Add the onions and garlic, cooking until they are fragrant and translucent, about 3-4 minutes.
4. Stir in the heavy cream, milk, and thyme. Bring to a gentle simmer, then remove from heat.
5. Layer half of the sliced potatoes, carrots, and parsnips in the prepared baking dish. Season with salt and pepper.
6. Sprinkle half of the cheddar cheese over the vegetables.
7. Pour half of the cream mixture over the cheese and vegetables.
8. Repeat with the remaining vegetables, cheese, and cream mixture.
9. Top with Parmesan cheese if using.
10. Cover with aluminum foil and bake for 45 minutes.
11. Remove the foil and bake for an additional 15-20 minutes, or until the top is golden and bubbly.
12. Let it cool for a few minutes before serving. Enjoy the joy of cooking from home!