How to Make It
In a large skillet, heat the vegetable oil until shimmering. Add the minced garlic, ginger and scallions and cook over moderate heat until softened, about 2 minutes. Add the black bean sauce, sherry, chicken broth and chile paste, bring to a boil and simmer the sauce for 1 minute.
Meanwhile, place the bok choy in a steamer and steam until crisp-tender, about 3 minutes. Add the watercress to the steamer and cook just until it wilts, about 1 minute longer.
Add the bok choy and watercress to the skillet with the sauce. Stir the cornstarch mixture; add it to the skillet and stir-fry over high heat until the sauce is thickened, about 1 minute. Transfer the vegetables to a bowl and serve with rice.