• Preheat the oven to 425 F degrees (220C)

For the filling

• Heat the oil in a small skillet over medium/low heat. Add the onions and cook till translucent - about 3 minutes. Add nutmeg, oregano, grated garlic and a pinch of red pepper flakes into the oil. Saute until fragrant, about 30 seconds. Add the spinach and mint and allow them to wilt, about 2-3 minutes. Sprinkle lightly with salt, mix and remove the mixture from the heat.
• Place the spinach mixture on a chopping board and coarsely chop the mixture. Mix in half the lemon zest then fold feta cheese with a cooking knife into the mixture. Keep some chunks of feta large. Set stuffing aside.
  

For the basting mixture

• Combine balsamic, oil, mustard, rosemary, lemon zest, garlic, salt, and pepper in a small bowl and whisk until smooth. Set aside.

Prep the tenderloin

• Place the tenderloin on a cutting board board and trim any remaining excess fat or tendons with a small sharp knife. Slice along the length just deep enough to butterfly it. Do not slice it too deep in case you pierce through the tenderloin.
  
• Place a plastic sheet on the tenderloin and flatten it with a mallet or small frying pan till it is about 1/4 inch thick.
  
• Spread the stuffing mix along the center but do not overstuff.
  
• Roll the tenderloin over the stuffing and tuck in the sides as you roll it.
  
• Tightly tie the tenderloin with cooking twine in four or five places about 2” apart so it holds its shape and the stuffing while it roasts.
  
• Line a roasting pan with parchment paper so that the basting liquid does not caramelize. Place the pork loin on that with the knots of string facing down. Smear all sides with the basting mixture. Use all the mixture because you will use it as a sauce once it is cooked. The balsamic vinegar mellows out and with the rosemary and garlic makes a delicious sauce.

  Baste occasionally while the pork is roasting.

Roast the tenderloin

• Roast the tenderloin for 15 minutes at 425F (220C) then reduce heat to 375F (190C)and roast for an additional 10 minutes or until a thermometer inserted in the thickest part of the loin reads at least 145F (63C). Allow the pork to rest for 10 minutes.
• Cut off the ties, slice the tenderloin into 1”thick slices and serve with a little of the cooked basting liquid.