Root Vegetable Gratin

• 4 Tbsp unsalted butter, divided, plus more for baking dish
• 1½ cups coarse fresh breadcrumbs
• 1½ cups grated Parmesan or Gruyère, divided
• 6 sprigs thyme
• Kosher salt and freshly ground black pepper, to taste
• 3 cups heavy cream
• 1 cup low-sodium chicken or vegetable broth
• 1½ pounds celery root, peeled, sliced 1/16" thick
• 1 pound rutabaga, peeled, sliced 1/16" thick
• 1 pound turnips, peeled, sliced 1/16" thick
• 1 pound potatoes, peeled, sliced 1/16" thick
• 2 garlic cloves, thinly sliced

Use turnips, rutabagas, kohlrabi, potatoes or a mix of these vegetables in this winter gratin. The key to gratins is having all the ingredients-whether they're basic potatoes or the mixed root vegetables-sliced the same thickness so they cook at the same rate. For this, a mandoline can help yield precise, even slices.If you use low-fat milk for a gratin, you will find quite a bit of liquid in the pan when you pull the dish from the oven. Let it sit for 20 to 30 minutes and the vegetables will reabsorb the moisture. If any liquid remains in the dish, it's delicious spooned over the gratin.Preheat oven to 400°. Butter a shallow 2-qt. baking dish. Melt 2 Tbsp. butter in a medium skillet over medium-high heat. Add breadcrumbs; cook, stirring often, until golden brown, 5-7 minutes. Transfer to a small bowl and toss with ½ cup cheese and 1 Tbsp. thyme leaves; season with salt and pepper and set aside.Bring cream, broth, thyme sprigs, and remaining 2 Tbsp. butter to a simmer in a medium saucepan over medium heat; season with salt and pepper. Remove from heat, cover, and let sit 10 minutes; discard thyme. Cover; keep warm.Toss vegetables in a large bowl; season with salt and pepper. Arrange 1/3 of vegetables in dish and top with ½ cup Parmesan. Repeat layers; top with remaining vegetables. Pour cream mixture over; place a piece of parchment paper directly on top of gratin.Bake until vegetables are crisp-tender and cream is thickened, 50-60 minutes. Uncover, top with breadcrumb mixture, and bake until gratin is bubbling and breadcrumbs are brown, 15-20 minutes longer. Let sit 10 minutes before serving.Make ahead: Up to 2 days; reheat at 325° until center is warm, about 30 minutes.