Mash soaked and drained fermented black beans with a fork and set aside.
Heat the oil in a small saucepan over medium-high heat. Add the garlic and ginger and cook for a minute or two until softened and very fragrant. Add the green onions and cook for another minute. Add the mashed beans and cook for another minute.
Add all remaining ingredients, except for the cornstarch mixture, and bring to a boil. Reduce the heat to medium and simmer uncovered for 10-12 minutes or until the liquid is reduced by half. Stir in the cornstarch mixture and simmer for another minute or until thickened.
Let the black bean sauce cool and store in an airtight container (preferably glass). Will keep for up to 2 weeks.Makes a little more than a cup.
For an additional touch of flavor (depending on what you're using it for) you can also add a 1/2 teaspoon or so of orange zest. Add it along with the broth and then proceed with the simmering.