• Preheat the oven to 425 degrees.

For the filling:

• Heat the oil in a small skillet over medium heat. Add the pepper flakes and grate the 3 cloves of garlic into the oil. Saute until fragrant, about 30 seconds. Add the spinach and allow it to wilt, about 2-3 minutes. Sprinkle lightly with salt then remove the mixture from heat. Place the spinach mixture in a paper towel and squeeze gently (be careful, it's hot!) to release any excess moisture. Place the spinach in a small bowl and stir in chopped tomatoes and feta cheese. Set aside.

For the topping:

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

To assemble:

• Slice the tenderloin down the middle lengthwise, leaving just about a 1/2 inch intact on one side. Open the top half and lay it flat. Spread the spinach mixture evenly down the center of the loin. Fold the top half back in place and use butcher's string to tie it together in 3-4 places (optional, see note). Place the pork loin in a roasting pan then smear all sides with the topping.
• Roast the tenderloin for 15 minutes, then reduce heat to 375 degrees and roast an additional 20-30 minutes or until a thermometer inserted in the thickest part of the loin reads at least 145 degrees. Allow the pork to rest for 10 minutes before slicing and serving.