Place 2 baking sheets in the oven and preheat to 425 degrees F.
Cut all the vegetables into 1 1/2-inch pieces. Cut the onions through the base core to keep some of the layers in chunky pieces. Toss all the vegetables with garlic, olive oil and salt in large bowl. Season generously with pepper.
Carefully remove the heated baking sheets from the oven, brush or drizzle with olive oil. Divide the vegetables evenly between the 2 pans, spreading them out to assure they don't steam while roasting. Roast the vegetables until tender and golden brown, stirring occasionally, about 45 minutes to 1 hour.
Some very large parsnips have a pithy core, which should be trimmed before cooking.