Peel and cut the potatoes in half. Put in a large pan and cover with cold water. Bring to the boil, add 1 tsp table salt and cook for 8 minutes. Drain the potatoes and shake well in the colander to fluff up the edges. Leave in the colander covered with a clean tea towel to absorb the steam for 5 minutes.
Meanwhile, put the goose fat in a roasting tin and heat for 5 minutes while the potatoes dry.
Mix the rosemary, garlic and sea salt in a bowl. Add the potatoes to the roasting tin, coat them in the hot goose fat, then sprinkle over the rosemary mix.
Roast in the oven for an hour, turning halfway through cooking, until very crisp and golden.