To make the caramelised onions: To a pan over medium-low heat, add the onions and olive oil and cook slowly until caramelised, about 40 minutes. Add the maple syrup, mix and remove from the heat.
To make the potatoes: Boil the potatoes in salted water until fork tender, about 15 minutes. Drain and let cool. Pass the potatoes through a ricer, or mash them by hand. Set aside.
To make the cherry glaze: Heat a pot over medium heat, and add the cherries, vanilla bean and port. Simmer over medium-low heat until saucy, 10 to 15 minutes.
To make the croquettes: In a deep pot, heat the duck fat to 190°C. In a bowl, mix the potatoes, butter, egg yolks, foie gras, caramelised onions, fleur de sel and pepper. Roll the mixture into balls (about the size of a golf ball), dip in the beaten egg and coat with panko. Fry the croquettes until golden, about 1 minute. Stab each croquette with a stick, dip in the cherry glaze and serve.