Start by peeling the potatoes and cutting them into small squares approximately 1cm x 1cm. Place into boiling salted water and boil for around 7 minutes.
Mix together the Onion Granules, Garlic Powder, Salt, Cumin and Cayenne Pepper to form a spice mix for the potatoes.
Drain the potatoes, and let them cool a little. Sprinkle a little bit of the rub over them and then put them in the fridge for 30 minutes.
Whilst the potatoes are cooling in the fridge, heat up the oven to 240°C, and place a high-sided baking tray in the oven with 4 tablespoons of the duck fat. You want your duck fat nice and hot before adding the potatoes.
After 30 minutes remove the potatoes from the fridge. Carefully add them to the hot duck fat, give them another small sprinkling of the rub, then cook for 3 minutes. Rotate the potatoes and cook for a further 4 minutes before removing them. Add them into a bowl with some kitchen paper to soak up some of the fat. Place them back in the fridge until you are ready to cook.
When you are ready to cook your potatoes, add 2 tbsp of the duck fat to a cast-iron skillet (or alternative pan), place on the BBQ and allow the duck fat to heat up. Add the potatoes and cook until the potatoes get nice and crispy.