Preheat the oven to 200C/400F/Gas6.
Parboil the potatoes for 10 minutes until the outside is slightly soft. Drain and place in a roasting tin flat side up. Use a fork to roughen the flat uppermost side.
Add the prepared parsnips, sprinkle with mixed herbs and arrange the onion rings over the top.
Drizzle with sunflower or olive oil and season with salt and lots of black pepper.
Roast for 40 minutes, basting several times, then add the cherry tomatoes and bake for a further 15-20 minutes until all the vegetables are cooked and the tomatoes have split open.