Preheat the oven to 200°C, gas mark 6. Peel the potatoes then cut into large, even-sized pieces. Rinse well under cold running water. Place into a large pan of cold, lightly salted water. Bring to the boil and simmer for 6-8 minutes. Drain well and return to the pan. Shake then heat gently for 1-2 minutes to dry them out a little then scatter over the polenta and shake again.
Meanwhile, place the goose fat or oil in a large roasting tin and preheat in the oven for 5–10 minutes. Carefully spoon the potatoes into the tray of hot fat. Add the sage and garlic then season. Gently turn to coat everything in the fat and spread the potatoes out as much as possible.
Cook for 50-60 minutes or until dark golden, turning halfway through. Season and serve.