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.