Preheat the oven to 425ºF. Oil a sheet pan or large baking dish and set aside.
Cut the carrots and parsnips into 3-inch lengths, making sure they are all roughly the same size. Cut away the cores from the parsnips.
Toss with the garlic, salt, pepper, rosemary, and olive oil until even coated.
Spread in an even layer in the prepared pan or baking dish. Cover with foil and place in the oven for 30 minutes.
Turn the heat down to 375ºF and uncover the pan. Stir gently, and continue to roast until lightly browned, 20 to 30 minutes. Remove from the heat, and serve.