Prepare the pork: Butterfly the pork tenderloin by slicing it lengthwise without cutting all the way through. Flatten with a meat mallet.
Make the stuffing: In a mixing bowl, combine breadcrumbs, mozzarella, sun-dried tomatoes, spinach, garlic, olive oil, Italian seasoning, salt, and pepper.
Stuff the pork: Spread the stuffing evenly on the flattened pork. Roll tightly and secure with toothpicks or kitchen twine.
Sear the pork: Heat a skillet with olive oil over medium heat. Brown the pork on all sides for 5 minutes.
Bake: Transfer the pork to a baking dish. Cover with foil and bake at 375°F (190°C) for 35-40 minutes.
Rest and slice: Let the tenderloin rest for 5 minutes. Remove toothpicks, slice, and serve hot.