Peel the potatoes, leaving the smaller ones whole and cutting the larger ones so that they are all roughly the same size. Put them in a large pan of cold salted water, bring to the boil and simmer gently for about 15 mins until cooked through. Tip gently into a colander and leave to drain and cool – there’s no need to shake. When they stop steaming, put them on a tray in a single layer and place in the fridge, uncovered, until ready to roast. Can be done up to two days ahead.
Once you have put the loin in to start roasting, spoon the goose fat into a flameproof roasting tin and heat on the hob for a few mins. Turn the heat off and, one at a time, turn each potato in the fat so they are completely coated. Make sure they are in a single layer and not touching. Once the oven has been turned down to 180C/160C fan/gas 4, put the potatoes in the oven with the pork and cook for 40 mins undisturbed.
Remove from the oven and, again, turn each potato. Return to the oven for 20 mins, then remove and repeat the turning process. The pork should have come out of the oven by now, so turn the oven up to 220C/200C fan/gas 7 and cook the potatoes for a final 20 mins. Add the rosemary and garlic 5 mins before the end. Once cooked, the potatoes should be deep golden and crunchy on the outside and fluffy in the middle. Sprinkle with sea salt and serve.