Cook rutabagas in pot of boiling salted water until tender, about 35 minutes.
Meanwhile, preheat oven to 400°F. Spray large rimmed baking sheet with nonstick spray. Combine oil, lemon juice, ginger, and sugar in large bowl. Add pears; toss to coat. Spread the cubed pear and sliced pear on prepared sheet. Roast until tender, turning pears every 10 minutes, about 35 minutes total. During the last 5 minutes add in the hazelnuts to roast.
Drain rutabagas; return to same pot. Mash to coarse puree. Stir over medium heat until excess moisture evaporates, 5 minutes. Add cream, butter. Mix in pears and any juices from baking sheet. Season with salt and pepper. DO AHEAD: Can be made 1 day ahead. Transfer to microwave-safe bowl. Cover; chill. Rewarm at 1-minute intervals, stirring.
Top with the sliced pear, fresh thyme, a drizzle of hazelnut oil and roasted hazelnuts.