Season the pork filet with celery salt, then set aside for 5 minutes. Combine the mashed potatoes, pickled onions, Parmesan and chives in a bowl and mix well. Form into four logs. Dip in flour, egg and breadcrumbs to coat.
Pour the extra virgin olive oil into a pan set over a moderate-high heat, then sear the pork on all sides for 3 minutes, turning several times. Add the butter and maple syrup, then continue cooking for 3 more minutes, until glazed. Set aside to rest.
Fry the potato croquettes in hot (180°C) vegetable oil for 3 minutes, until golden and crisp, then drain on kitchen paper. Carve the pork then serve with croquettes, baby beetroot, soy beans, jus and nasturtium leaves.