For the roasted vegetables: Put 2 baking sheets in the oven and preheat to 425 degrees F.
Cut the parsnips, squash and beets into 1 1/2-inch pieces and put in a large bowl. Cut the onion through the stem into chunky pieces and add to the bowl along with the carrots and garlic. Add the olive oil, 1 1/2 teaspoons salt and a generous amount of pepper and toss together.
Carefully remove the hot baking sheets from the oven and brush or drizzle with olive oil. Divide the vegetables between the baking sheets, spreading them out to assure they don't steam while roasting. Roast the vegetables, stirring occasionally, until tender and golden brown, 45 minutes to 1 hour.
For the gremolata: Meanwhile, combine the mint, parsley, olive oil and lemon zest and juice in a small bowl. Season with salt and pepper.
Transfer the vegetables to a serving platter and drizzle with the gremolata.
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