Place a large skillet over medium-high heat to get hot. Drizzle oil over the pork tenderloin and season well with salt and pepper. Carefully put the pork tenderloin in the skillet to sear until brown on all sides, about 3 minutes per side.

Place the sliced onions in the bottom of a slow cooker. Place seared pork loin on top of onions.

Sprinkle the garlic and brown sugar over the pork tenderloin.

Mix the water, balsamic vinegar, and soy sauce. Pour over the top of the pork loin.

Cover and cook on high for 2 to 3 hours or low for 6 hours or until it reaches an internal temperature of 145 degrees. 

Transfer the juices to a medium saucepan. Bring the mixture to a boil and simmer until syrupy, about 15 minutes.  Pour over the pork and serve.