Place the chicken thighs in a large dish, pour over the piri-piri sauce, mix well to coat, then leave to marinate for 15 minutes.
Preheat the oven to 200 degrees C (390 degrees Fahrenheit).
Peel the baby potatoes, carrots and parsnip, give them a good rinse, then cut the parsnip and carrots in chunks.
Bring a pan of salty water to the boil and add the vegetables, letting them simmer for 5 minutes.
Drain the water, then arrange the chicken thighs and vegetables on a large baking tray.
Scatter around the unpeeled garlic cloves, dried rosemary, drizzle the olive oil over the vegetables, and season with salt and pepper.
Bake for about an hour, making sure you flip the chicken thighs and vegetables over half the way through. You can also add the onion at that point, cut in wedges.