**Prepare the Turnips**
Preheat your oven to 375°F (190°C). Peel and thinly slice the turnips, aiming for slices about 1/8-inch thick. In a large pot, bring water to a boil and add the turnips. Cook for 5-7 minutes, or until just tender but still firm. Drain the turnips well and set them aside.
**Make the Creamy Sauce**
In a skillet, melt 2 tablespoons of butter over medium heat. Add the minced garlic and cook for about 1 minute until fragrant. Pour in the heavy cream and milk, and stir in the thyme. Bring the mixture to a simmer, then season with salt and pepper to taste. Let the sauce simmer for 3-5 minutes, allowing it to thicken slightly.
**Assemble the Gratin**
In a 9x13-inch baking dish, layer the cooked turnip slices, slightly overlapping them. Pour half of the creamy sauce over the turnips, ensuring they are evenly coated. Sprinkle half of the cheddar and Gruyère cheese on top. Repeat with another layer of turnips, followed by the remaining sauce and cheeses.
**Prepare the Topping**
In a small bowl, mix the panko breadcrumbs with the melted butter and Parmesan cheese (if using). Sprinkle the breadcrumb mixture evenly over the top of the gratin for a crispy, golden finish.
**Bake**
Place the dish in the oven and bake for 25-30 minutes, or until the top is golden brown and the sauce is bubbling. For an extra crispy top, broil for an additional 2-3 minutes, watching closely to avoid burning.
**Serve**
Remove the gratin from the oven and let it sit for a few minutes to set before serving. Garnish with additional thyme or parsley if desired.