Place 2 tablespoons fermented black soybeans in a small bowl and cover with water. Let soak for 10 minutes. Meanwhile, trim and cut 2 medium bell peppers into 1-inch squares. Thinly slice 1 medium scallion (about 2 tablespoons). Finely chop or grate until you have 2 tablespoons of garlic and place in a medium bowl. Add 2 tablespoons of vegetable oil and 2 tablespoons of cornstarch to the bowl.
Drain the soybeans and mash with a mortar and pestle, or place on the cutting board and mash with the back of a spoon. Add to the bowl with garlic and stir to combine. Cut 1 pound of flank steak into 2-inch wide pieces. Thinly slice each piece across the grain, add to the garlic mixture, and toss until evenly coated.
Heat a 14-inch flat-bottom wok or 12-inch skillet over high heat until a bead of water vaporizes within 1 to 2 seconds of contact. Add the steak mixture and spread into a single layer. Cook undisturbed until the meat begins to sear, about 1 minute. Stir-fry until the meat is lightly browned but not cooked through, about 1 minute more.
Reduce the heat to medium-high. Add the bell peppers and stir-fry until starting to soften, about 1 minute. Drizzle in 3/4 cup of the chicken stock slowly, and stir-fry until the beef is just cooked through and the sauce is thickened, about 1 minute. Add 1/4 cup more stock for more sauce if desired. Garnish with the scallions and serve over cooked white rice.