1. Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius).
2. Place the corned beef into a 9×13-inch pan.
3. Mix balsamic vinegar, water, and brown sugar.
4. Pour over corned beef.
5. Cover with aluminum foil, place on middle rack of oven and bake for three hours.
6. When corned beef is ready, remove from oven.
7. Allow to sit for 15–20 minutes.
8. Pull a fork over the top of the corned beef and it will begin to pull apart in a stringy fashion.
9. Transfer the pulled corned beef into a medium-sized saucepan.
10. Add the sweet roasted onions.
11. Dissolve two teaspoons of potato starch in 1/4 cup water and pour over corned beef and onion mixture.
12. Over a medium flame, stir the mixture as the liquid heats up and thickens.