Put the potatoes into a large pan, and the parsnips and celeriac into another pan.
Cover both pots of veg with cold water, add a pinch of salt and bring to the boil. Cook the potatoes for 6 minutes and the parsnips and celeriac for 4 minutes. Drain and shake in a colander to roughen the edges.
Heat the oven to Gas Mark 6, 200ºC, 180ºC fan.
Put the goose fat in a very large roasting tin and heat in the oven for 4 minutes until sizzling hot. Put all the vegetables into the tin, making sure they are spread out and covered well in fat, so they roast evenly.
Roast for 1 hour, turning after 30 minutes.