Preheat oven to 350°F (180°C) (I used a convection oven). Lightly grease wire rack and place inside roasting pan. Add a cup of water to pan.
Melt the butter in a large frying pan over medium heat until bubbling. Add onion & cook for 5 minutes or until softened. Remove from heat.
Add breadcrumbs, apricots, pecans, parsley, salt & pepper; stir until well combined.
Place pork on cutting board. Using a sharp knife, cut pork horizontally (knife will be parallel to the board) about 1 inch below the fat on top. Cut almost all the way through to the other side. Open pork out flat. Cut thick section in the same manner but now you’re cutting it to unfold in the opposite direction. So the opened fat top should be on your right, this will open up to your left. See diagram. You will have a rectangular piece in front of you that appears to have three creases. Cover with plastic wrap & pound with a meat mallet (or other heavy, unbreakable object such as a cast iron pan) until it’s an even thickness & shape. If you’ve made the cuts correctly, there shouldn’t be too much pounding. Now, if this is all too much to follow, then cut the pork horizontally through the center almost through to the other side – this method will mean you can’t roll the pork but will have to simply fold the top over the bottom.
Spread the mustard evenly over pork. Spread the stuffing over the mustard leaving a 1-inch border down the long fat side. Roll up pork firmly from short end without the fat. Have the seam facing down & the fat facing up. Carefully score the fat: using a sharp knife cut diagonal lines one way then the opposite way, like you’d do with a ham. Rub a little olive oil, salt & pepper over the pork.
Secure pork with butcher’s twine tied at 1-inch (2.5cm) intervals. Place in prepared pan. Roast, uncovered, in preheated oven for 1 1/2 to 2 hours or until meat thermometer inserted into center of roast reads 150°F (65°C), do not overcook. Add a little extra water to the pan if the pan is over browning.
Remove from oven and cover with foil. Reserve the pan drippings. Let pork stand for 20 minutes before cutting into 3/4 – 1-inch (2 – 2.5cm) thick slices. Serve with gravy.