Pre-heat the oven to 190°C.
Peel and halve the parsnips down the middle, splitting them in half. Peel and cut the red onions into large chunks. Wash and drain the potatoes, making sure any dirt or grit has been washed away.
Cut the pumpkin in half through its centre axis, then remove any seeds and stringy pulp. Turn the halves on their sides so that they are stable; cut them into 2cm thick slices. Arrange the prepared vegetables between two shallow roasting trays. Drizzle evenly with the sunflower oil and season each tray of vegetables with a tsp of dried thyme and rosemary.
Sprinkle over a tsp of caster sugar and season generously with salt and pepper. Toss the vegetables well to coat them as evenly as possible in the seasonings. Cover the trays loosely with aluminium foil and roast for 45 minutes. Remove and check to see if the vegetables are tender.
If they are, remove the foil, increase the oven to 220°C and roast uncovered for a further 10-15 minutes. If they aren't tender just yet, return to the oven covered with the foil, checking every 5 minutes to see if they are tender. Remove the vegetables after they have had 10-15 minutes in the oven uncovered. Serve immediately.