Preheat the oven to 180ºC, gas mark 4. Tip the parsnips into a large roasting tin. In a small bowl, whisk together the oil, mustard and honey. Season and pour over the parsnips, toss to coat, then roast for 15 minutes.
Increase the oven temperature to 220ºC, gas mark 7, then add the carrots and toss again until everything is coated. Return to the oven for 45-55 minutes until golden and tender, turning everything halfway through. Serve with a scattering of thyme leaves and pomegranate seeds, if liked.