Heat oven to 200°C (fan 180°C / gas 6). Bring a large pan of water to the boil and cook the parsnips 1 minute, then the carrots 1 minute, then the cauliflower 1 minute. Drain, reserving a cup of the cooking water.
Put the parsnips, carrots and red onion in a roasting tin and drizzle with olive oil. Roast 15 minutes.
Add the cauliflower, dried coriander, coriander seeds and lemon juice. Toss and roast 10 minutes.
Blend the yogurt, garlic, turmeric, honey, mint and lemon juice to a smooth sauce. Loosen with a little reserved cooking water until drizzling consistency; season to taste.
Transfer veg to a platter, drizzle with sauce and scatter over mint and pomegranate seeds.