Preheat the oven to 200C/400F/Gas 6.
Place the parsnips on a roasting tray with half the olive oil and toss to coat.
Roast in the oven for 15 minutes until golden-brown and tender, then remove and set aside.
Heat the remaining olive oil and the butter in a large saucepan. Add the onion and dried rosemary, cover and fry gently for 3-4 minutes.
Add the roasted parsnip and the remaining ingredients. Bring to the boil.
Reduce to a simmer and cook for five minutes, or until the parsnip is very soft.
Transfer to a blender and pulse until smooth.
Season to taste with salt and freshly ground black pepper.