Pat the pork tenderloin dry with paper towels. Remove the silver skin.Note: The silver skin is a band of connective skin that will remain tough and chewy after cooking. It is a white band of connective tissue on one side of the pork.
Combine brown sugar, salt, garlic powder, thyme, paprika, black pepper, and red pepper flakes in a small bowl.Rub spice mixture all over pork.
Heat oil in a large skillet over medium-high heat. Add pork and sear on all sides.
Transfer pork to a 6-quart slow cooker.
In a medium bowl whisk together chicken broth, honey, balsamic vinegar, ketchup, Worcestershire sauce, garlic and rosemary.Pour over pork.
Cover and cook on LOW for 4 to 5 hours or on HIGH for 2 1/2 hours.
Transfer cooking liquid to a saucepan. Whisk together cornstarch with 1/4 cup cold water. Add to saucepan. Bring to a boil and whisk until thickened.
Slice pork and serve with sauce.
Notes
Use an instant read thermometer to make sure pork reaches an internal temperature of 145 degrees F.