How to Make It
Snap off the tough ends of the asparagus and discard them. Cut the asparagus into 1-inch lengths. In a large pot of boiling, salted water, cook the asparagus for 3 minutes. Drain. In a small bowl, combine the cornstarch and water.
In a wok or large frying pan, heat the oil over moderately high heat. Add the garlic. Stir-fry until fragrant, about 30 seconds. Add the onion and stir-fry until softened, about 3 minutes. Add the sliced mushrooms and stir-fry until brown, about 5 minutes.
Stir in the stock, soy sauce, 3 teaspoons of the vinegar, the sesame oil, and the salt. Simmer for 2 minutes. Stir the cornstarch mixture, add it to the pan, and simmer until the sauce has thickened. Add the tofu and simmer for 3 minutes. Add the asparagus and simmer until hot, about 2 minutes longer. Stir in the remaining 2 teaspoons vinegar. Serve topped with the sesame seeds.