Combine brown sugar, water, balsamic vinegar, soy sauce, and cornstarch in a small saucepan over medium heat. Cook, stirring constantly, until mixture thickens, 4 to 6 minutes. Remove from the heat and set aside.

Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking sheet.

Combine garlic, sage, salt, and pepper in a small bowl. Rub garlic mixture over pork tenderloins.

Heat oil in a large skillet over medium-high heat. Arrange tenderloins in the skillet and brown evenly, 2 to 3 minutes per side. Transfer pork to the prepared baking sheet and brush with the balsamic glaze.

Bake in the preheated oven for 20 minutes. Turn pork, brush again with glaze, and continue cooking until pork is slightly pink in the center, about 20 minutes more, or until desired doneness. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C).

Remove tenderloin to a platter. Allow to rest for 10 minutes. Drizzle a little extra glaze over the top. Carve and serve with remaining glaze on the side.