Preheat the oven to 220°C/430°F (without fan).
Place 3-4 tablespoons of duck fat (or vegetable oil) into a large roasting pan. Depending on the size of your roasting pan, you may need more or less fat.
Place the roasting pan into the oven to heat while you prepare the potatoes.
Peel and wash the potatoes.
Cut the potatoes into large chunks, ideally by cutting a triangle from the middle of the potato.
Place the chopped potatoes into a large pot filled with cold water.
Bring the pot to a boil.
Once the water is boiling, add the sea salt.
Cook the potatoes for about 10 minutes (depending on how large you have cut the potatoes). You don't need the potatoes to cook through fully at this stage.
Drain the potatoes well.
Return the potatoes to the pot, which should still be hot.
Place a lid on the pot.
Shake the pot gently to roughen the edges of the potatoes. You want the edges of the potatoes to just slightly soften.
Remove the roasting pan from the oven.
Use a pair of tongs to place the potatoes, one at a time, into the hot fat. Turn each piece of potato so that it is coated on all sides in the fat.
Return the roasting pan to the oven.
Roast the potatoes for about 1-1.5 hours, or until they are golden and crispy to your liking. Turn the potatoes over once or twice during this time.
To serve, sprinkle immediately with sea salt, plus a light garnish of chopped parsley.