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.If you want the sauce to be sweet and spicy, add 1/2 teaspoon of crushed red pepper flakes or cayenne pepper to the sauce.