A hint of curry adds interest and depth to this pairing of winter vegetables. Although parsnips and carrots may be humble roots, this dish is rich and satisfying. Turnips and rutabagas can also be added or substituted; just be sure to steam the vegetables until crisp-tender before baking.
Position a rack in the middle of the oven and preheat the oven to 375 degrees F.
Steam the carrots over high heat for 10 minutes, then add the parsnips to the carrots and continue steaming until the vegetables are crisp-tender, 5 to 8 more minutes. Remove from the pan and set aside.
Meanwhile, heat the oil in a large skillet over medium heat. When the oil is hot, add the sliced onions and cook, stirring frequently, until the onions are soft and have begun to caramelize, about 15 minutes. Stir in the curry powder and cook another 1 to 2 minutes. Add the carrots and parsnips and season to taste with salt and pepper.
Transfer the vegetable mixture to a 7x11-inch baking pan. Pour the heavy cream over the vegetables.
In a small bowl, combine the bread crumbs, butter, and parsley, and stir to blend. Sprinkle the bread crumbs over the top of the vegetables and transfer the dish to the preheated oven. Bake until the cream is absorbed and has begun to bubble along the edges, 30 to 40 minutes.