Preheat the oven to 180°C. Cut the parsnips into 5 cm. long pieces; slice the pieces across into 1 cm. slices and trim the sides. Slice into 1 cm. thick batons. Heat the olive oil in a cast-iron pan and add the parsnips. Season with salt and freshly ground pepper and cook until they are evenly caramelised. Transfer the pan to the oven and continue to cook for a further 4–5 minutes or until the parsnips are tender. Turn the parsnips so that they become evenly coloured. Add the butter, thyme and garlic for the last 2 minutes of cooking. Remove the parsnips from the oven, drain and remove the thyme and garlic. Adjust the seasoning if necessary.