1. Drain the beans, and save the liquid from the can in small bowl.
2. Pour 1/2 cup bean liquid into the saucepan.
3. Add the cornstarch in the bean liquid. Stir well.
4. Add the vinegar and brown sugar.
5. Put on medium heat and bring to a boil.
6. Turn the heat to low.
7. Add the green beans and onions. Heat and serve.