Peel and chop the potatoes into large chunks, roughly the size of a golf ball.
Parboil the potatoes in a pan of boiling, salted water for 10 minutes, then drain in a colander, shaking it to chuff up the edges.
Meanwhile, peel long strips of peel from the oranges using a speed-peeler, and pick the sage leaves.
Place the goose fat or oil into a large roasting tray (you may need to use 2 trays) on the hob over a low heat, add the unpeeled garlic, orange peel and sage leaves, then fry for 30 seconds.
Add the spuds and toss together until well coated.
Place the tray in the oven and cook for 45 minutes, or until golden and crisp.