Prepare the eggplant: Wash, remove the stem, slice crosswise in 2-inch pieces, then slice into thick long strips (about 2 inches long).
Transfer the eggplant to a large bowl for marinating. Sprinkle it with a generous amount of salt and let it marinate for 10 minutes to draw out excess water. This step is needed to reduce its spongy texture and make it creamy and crisp. It also helps lessen the oil absorption during the stir-frying step.
After marinating, rinse off the excess salt with water. Then, drain as much water as you can and squeeze out any remaining excess water from the eggplant with both hands.
In the meantime, make the sauce: Combine water, Chinese black vinegar, light soy sauce, sugar, and cornstarch. Mix well and set aside. At this step, you can go ahead with chopping the other fresh vegetables, such as ginger, garlic, green bell pepper, red bell pepper, and green onions.
Heat oil in a pan and stir-fry minced meat until light brown.
Mix in minced ginger, garlic, and doubanjiang. Continue stirring until the seasonings are well-mixed and the minced meat takes the color of the doubanjiang (slightly red).
Next, add the chopped eggplant and the prepared sauce, stir well, and cook until the flavors meld. Remember to mix the sauce once more before pouring it into the pan since the cornstarch tends to settle at the bottom of the bowl.
Finally, add thinly sliced green and red bell peppers and chopped green onion. Stir-fry briefly over high heat, and you're done! I recommend chopping the bell peppers and onions ahead of time to avoid over-frying the eggplant. If the eggplant is cooked too much, it will turn soggy and lose its bright purple color.