Peel the carrots, parsnips and butternut squash and cut lengthways into 7–8cm pieces. Put in a large freezer bag with 1 tsp cumin seeds, the lemon zest, rosemary and olive oil. Shake together in the bag and chill overnight to marinate.
When ready to cook, preheat the oven to 180°C, fan 160°C, gas 4. Tip into a roasting tin and cook for 30 mins or until golden and tender.