Slice 2 pounds pork tenderloin lengthwise about ⅔ of the way through, keeping it intact on one side. Use 1 teaspoon kosher salt and ½ teaspoon ground black pepper to season the pork inside and out.
Carefully stuff 1 cup fresh spinach leaves, ¼ cup cooked and crumbled bacon, ¼ cup shredded parmesan cheese, and 2 teaspoons minced garlic evenly down the slit in the pork, then close the tenderloin back up as best you can.
Wrap the tenderloin tightly with kitchen twine (also called butcher's twine) to prevent it from opening up while it roasts. You can tie off the end or simply tuck the end of the twine securely into itself.
Rub the outside of the pork with 2 tablespoons softened unsalted butter and cook for 35-40 minutes, until the center of it reaches an internal temperature of 145 degrees Fahrenheit.
Let the pork rest for five minutes before removing the twine, slicing, and serving.