Pre-heat the oven to 180C (350F).
Cut the pork loin down the middle lengthways so it’s open like a book (don’t cut right the way though it should still be one whole piece). Place a piece of plastic wrap over the pork and bash the pork with a meat mallet or rolling pin a few times all over to tenderise the meat and make it an even thickness. Discard the plastic wrap.
Using a mezzaluna or knife finely chop the lemon zest and rosemary together.
Sprinkle the pork with sea salt flakes (you can also use regular salt) then sprinkle over the lemon zest, rosemary, grated garlic, oregano, red pepper flakes and a pinch of black pepper. Finally, drizzle the white wine and olive oil (1 teaspoon) over the pork and rub everything together.
Tightly roll the pork lengthways from one end to the other and secure it with kitchen string. You can do this by tying loops of string at each end then about ½ an inch apart across the pork (see our step-by-step photos and recipe video for tying up the pork using butcher's knots, see note 3).
Once the pork is secure with kitchen string sprinkle more sea salt flakes and fennel seeds over the top of the meat and drizzle with 1 tablespoon of olive oil and rub it all over.
If you have time place the pork in the fridge for 2 hours to overnight to infuse the flavours but it can be roasted right away.
Thickly slice the onion and place in a roasting tin as a bed for the pork to sit on. Place the pork on top and cover with the baking parchment.
Roast for 30 minutes covered then remove the baking parchment and roast for another 40 minutes uncovered.
Remove from the oven, cover with foil and let it rest for 20 minutes before slicing and serving.