Position oven racks in top and bottom thirds of oven; preheat to 425°F (220°C).
Whisk olive oil, thyme and rosemary in large bowl. Add sweet potatoes, beets, carrots, turnips, fennel and onions; toss to coat.
Divide between two rimmed baking sheets. Roast vegetables until tender and brown in spots, turning occasionally, about 30 minutes. Add walnuts to vegetables and continue to roast for 10 minutes. Remove from oven; immediately sprinkle with salt and pepper.
In small bowl, whisk together walnut oil, vinegar, orange zest, orange juice, salt, pepper and parsley.
Arrange roasted vegetables on a large platter and drizzle with vinaigrette. Serve hot or at room temperature.
Per serving: about 240 calories, 5 g protein, 16 g fat (2 g saturated fat), 22 g carbohydrates, 6 g fibre, 0 mg cholesterol, 390 mg sodium. %RDI: 6% calcium, 8% iron, 240% vitamin A, 30% vitamin C.