In a small bowl combine sage, salt, pepper, and garlic. Mix well. Rub the mixture over the tenderloin.
Add a ½ cup of water to the slow cooker. Place the tenderloin into the slow cooker. Cook on low for 6 to 8 hours.
An hour before the tenderloin has finished cooking. In a small saucepan combine brown sugar, cornstarch, balsamic vinegar, ½ cup water, and soy sauce. Heat over medium heat while stirring until the mixture becomes thick.
Brush the tenderloin in the slow cooker 2 to 3 times during the last hour of cooking.
Optional step: To get a caramelized crust—remove the tenderloin from the crockpot and place onto a baking sheet lined with aluminum foil. Brush the tenderloin with the glaze and set under the broiler for 1-2 minutes or until bubbly and caramelized. Repeat 2 to 3 more times until the desired crust is achieved.