In a small bowl, combine the sage, salt, pepper and garlic.
Place the tenderloin in the slow cooker. Rub the seasonings over the pork.
Pour ½ cup water in the slow cooker, on the edge so that you don't wash off all the spices you just rubbed on.
Cook on low for 6-8 hours, the preheat your broiler.
While the roast is finishing up in the slow cooker and your broiler heats up, combine the ingredients for the glaze in a small sauce pan: brown sugar, cornstarch, balsamic vinegar, water, soy sauce.
Heat over medium and stir until mixture thickens, about 4 minutes.
Line a baking sheet with aluminum foil and spray with oil, or use a silpat. Remove the pork from the crock pot and place on the lined baking sheet. Brush a generous amount of the glaze on the pork. Put your oven rack as high as it will go, and broil the pork for 1 or 2 minutes, until bubbly and caramelized. Don't walk away! Repeat 2 to 3 more times until it is as crusty as you want it. I wouldn't go many more times than this though, or your pork will dry out.
Serve with remaining glaze on the side.