Preheat the oven to 180°C/160°C fan/gas mark 4. Quarter the parsnips and halve the carrots, before placing in a large roasting tin along with the onions.
Mix together the rest of the ingredients and pour over the vegetables. Season well, then toss together so everything is evenly coated.
Roast in the oven for 20 minutes, turn, and then roast for another 20 minutes.