- Make sauce: In glass measuring cup, mix all ingredients. Set sauce aside.
- In mortar, combine garlic, black beans, ginger and chili paste. Mash to paste with pestle. (If you don’t have a mortar and pestle, combine ingredients in small bowl and work into paste with a fork.) Set aside.
- Quarter fennel, remove core and thinly slice. Set aside. Remove tough core from base of bok choy, then chop horizontally, cutting through stems and leaves. Set aside separately from fennel.
- In wok or large heavy skillet, heat 2 teaspoons oil over high heat. Add fennel, cover and cook until softened, about 3 minutes, uncovering and stirring occasionally. Transfer to bowl.
- Heat 1 teaspoon oil in wok over high heat. Add bok choy, cover and cook until softened, 2 to 3 minutes, uncovering and stirring often. Add to bowl with fennel.
- Heat remaining 1 teaspoon oil in wok over high heat. Add black bean mixture and stir-fry until fragrant, 1 to 2 minutes. Return fennel and bok choy to wok and stir to coat. Add reserved sauce and stir well.
- Add cornstarch mixture and cook, stirring, until mixture is slightly thickened, 2 to 3 minutes. Add watercress and toss until wilted, about 1 minute. Serve hot.