Peel and halve the potatoes (quarter any large ones; the pieces should all be around the same size). Put the potatoes into a large saucepan along with the rosemary, garlic, bay and peppercorns. Cover with cold water and bring to the boil. Reduce the heat slightly and simmer for 5-7 minutes.
Drain the potatoes and leave in the colander to steam dry for 5-10 minutes.
Meanwhile preheat the oven to 220C/200C Fan/Gas 7. Put the goose fat into a large roasting tin and put in the oven for 3-5 minutes, or until very hot.
Carefully tip the potatoes (as well as the aromatics they were boiled with) into the hot goose fat, shake gently to move around and roast for 35-40 minutes, turning once or twice.
After 20 minutes in the oven remove and gently squash the potatoes a little to create more surface area to make them extra crisp. Return to the oven and cook for a further 15-20 minutes, or until crisp and golden-brown.
Discard the aromatics and then season the potatoes with salt. Serve.