Add the duck fat to a deep-sided roasting tray and place in the oven to heat up for 20 minutes.
Bring a large pot of well-salted water to the boil over high heat. Add the fries and cook for 2 - 3 minutes until starting to become a little tender.
Drain and pat dry. Toss in a little salt then add them to the hot oil.
Return pan to the oven for 30 - 35 minutes until golden and crispy.
For the onions, add the butter to the pan along with the sunfl ower oil.
Fry the onions for 20-25 minutes on a low heat until they're a little caramelised but also crispy. Set these aside and keep warm.
In the same pan, make the sauce. Add the sunflower oil to the pan along with the garlic, ginger and curry powder and fry for a minute until the garlic and ginger have softened.
Stir through the peanut butter and soy sauce along with the coconut milk. Simmer for 2 - 3 minutes until thickened.
Serve the fries topped with satay sauce, mayonnaise, onions and coriander leaves.