Place chopped onion into the slow cooker.
Put pork on top of the onions.
Combine chicken broth, brown sugar, balsamic vinegar, garlic, soy sauce, salt, pepper in a small bowl.
Pour liquid over the pork.
Cook on high for 4-5 hours, or on low for 7-8 hours.
Remove pork and shred using two forks.
Strain remaining sauce to discard any fat and onion pieces.
Place sauce in a medium sized sauce pan. Bring to a boil.
Whisk together 1 TBS cold water and 1 TBS corn starch and add into the sauce.
Add additional brown sugar and balsamic vinegar to the sauce, and stir together.
Continue to cook for 3-4 minutes until slightly thickened.
Pour sauce over shredded pork and serve.