In a medium skillet, heat 2 tablespoons of olive oil over medium heat. Add minced garlic and red pepper flakes, sauté until fragrant. Mix in spinach until wilted, then drain and combine with sundried tomatoes and feta cheese.

Preheat oven to 425°F. Butterfly the pork tenderloin, spread filling inside, and secure with kitchen twine.

In a bowl, whisk balsamic vinegar, olive oil, Dijon mustard, minced rosemary, lemon zest, and season with salt and pepper.

Brush glaze over the pork and roast for 15 minutes at 425°F, then reduce temperature and roast for an additional 20-30 minutes until internal temperature reaches 145°F.

Let the pork rest for 10 minutes before slicing to reveal the filling, then serve.