To make sauce: In bowl, stir together stock, black bean sauce and cornstarch; set aside.
Drain and pat tofu dry; cut into 1-inch (2.5 cm) cubes. In large skillet or wok, heat vegetable oil over medium-high heat; stir-fry white parts of green onions, garlic and ginger for 20 seconds. Add asparagus, red pepper and tofu, adding more oil if necessary, stir-fry for 3 to 5 minutes or until vegetables are tender.
Stir in sauce and hot pepper flakes (if using); bring to boil and stir until thickened and tofu and vegetables are coated with sauce, about 1 minute. Sprinkle sesame oil over top. Serve over hot rice noodles. Sprinkle with green parts of green onions.