Preheat oven to 375˚F. Line baking sheets with parchment, and set aside.
Combine the parsnips, turnips, carrots, and sweet potatoes in a large bowl. Toss with 6 tablespoons of the olive oil and the salt and pepper. Spread onto the sheet pans in a single layer and roast until tender, about 25 minutes. Toss the onion with the remaining 2 tablespoons of olive oil and place on a baking sheet. Roast about 10 minutes, until golden and tender.
Heat the butter in a large sauté pan over medium-high heat. Add the leeks with a pinch of salt and pepper and turn the heat to low, cover, and cook, stirring occasionally, until tender and wilted, about 15 minutes.
Combine the roasted vegetables in a large bowl, and fold in the leeks. Drizzle with the warm maple syrup, and toss to coat. Serve.