Arrange the slices of ham close together on a clean cutting board, with the sage leaves on top.
Trim the sinew off the pork fillet and place it on top of the ham.
Cut a slit along the length of the meat and spread the mustard inside.
Put the block of feta in the cavity of each mushroom and arrange them in a row inside the fillet.
Fold the meat over the mushrooms, then wrap the ham around the fillet to form a neat roll.
Sear the meat roll quickly in a pan over high heat until sealed all over and nicely browned.
Allow to cool slightly.
Open up the roll of puff pastry on a clean work surface, then roll the dough slightly thinner.
Place the seared fillet on the dough, then wrap the dough tightly around the meat so that the ends overlap just enough to seal it.
Trim off any access dough, pinch the ends closed and place the roll with the seam side down on a greased baking tray.
Brush all over with egg.
Decorate with the left over dough, also brush this with egg and place the pie in the fridge while the oven is preheating.
Preheat the oven.
Bake the pie 35-40 minutes or until the dough is golden brown and the meat is cooked through.
Rest for about 10 minutes before slicing.
Swap the mustard in the filling with onion marmalade for a sweeter flavour profile or smear a spoonful over the seared meat before it is wrapped.
Don’t be too generous or it could make the pastry soggy.