Place the oil in a large frying pan over medium-high heat. Season pork and cook for 5 minutes, turning, until browned all over. Transfer to a plate to cool. Pat dry with paper towel and brush with mustard.
Reduce the heat to medium and, to the same pan, add butter, swirling to melt, then add shallot and fennel seeds and cook, stirring regularly, for 2 minutes or until softened. Add spinach and cook for a further minute or until just wilted. Spoon onto a plate lined with paper towel and leave to cool slightly, then squeeze out excess liquid.
Place a sheet of baking paper on a work surface and place the prosciutto slices on top and overlapping in a rectangle the length of the pork fillet and wide enough to enclose the meat.
Spread the spinach mixture evenly over the prosciutto, leaving a 2cm/1” border. Place pork fillet on edge of the prosciutto closest to you and, using baking paper to help, roll into a cylinder to enclose.
Place the 2 sheets of puff pastry on the work surface overlapping slightly. Place the prosciutto-wrapped pork on one long side and brush the pastry edges with egg wash. Roll the pork and pastry into a snug cylinder and place seam-side down on a baking tray lined with baking paper. Trim any excess pastry and pinch ends together to seal.
Brush all over with egg wash, score diagonally with a sharp knife and scatter with ½ tsp salt flakes and extra fennel seeds and chill for 30 minutes.
Preheat oven to 220°C/425°F and bake pork for 30-35 minutes until pastry is golden brown. Rest for 10 minutes. Thickly slice to serve.
