Cut the potatoes into diced blocks (2.5-5cm), put into a saucepan, with plenty of cold salted water, bring to the boil and allow to simmer for about 5 minutes.
Drain the potatoes in a colander – remove as much water as possible.
Melt the duck fat in a saucepan, then add the potatoes, garlic, and rosemary. Ensure potatoes are well coated in fat.
Place potatoes in a roasting pan, in a single layer. Roast the potatoes for 15 mins, then take them out of the oven and turn them over. Roast for another 15 mins and turn them over again. Put them back in the oven for another 10-20 mins, or however long it takes to get them golden and crisp.
Serve with the cracked pepper, sea salt and thyme is you like.
You can also cook your other roast vegies in the same way for a wonderful taste and crisp presentation.