Tip the potatoes into a large pan of cold salted water and bring to the boil. Turn the heat down slightly and keep the water bubbling gently for 3 mins, then add the parsnips and continue to simmer for 3 mins more. Drain everything and leave until cool enough to handle, then separate the parsnips and the potatoes.
Heat oven to 200C/180C fan/gas 6 with a large roasting tin containing the goose fat inside. When the goose fat is hot, remove the tin from the oven. Carefully tip in the potatoes and turn them so they’re completely coated in fat. Place the tin back in the oven and leave undisturbed for 1 hr. Remove the tin from the oven, add the parsnips and gently turn everything together.
Increase oven temperature to 220C/200C fan/gas 8. Roast everything for 20 mins, then turn the parsnips and potatoes again with the rosemary, if using, and roast for about another 15 mins until everything is golden and crisp. Sprinkle with sea salt and scoop into a warm serving dish.