Watercress makes a colorful bed for the shrimp. Cover and refrigerate for 30 minutes. Mix remaining 2 tablespoons Sherry, chicken stock, soy sauce, catsup, cornstarch, rice vinegar, sugar, sesame oil and cayenne pepper in small bowl. Heat 2 teaspoons peanut oil in wok or heavy large skillet over high heat. Add walnuts and stir-fry for 1 minute.
Transfer walnuts to plate using slotted spoon.
Add watercress to wok and stir-fry until just wilted, about 1 minute. Divide watercress among plates. Add 2 teaspoons peanut oil, bell peppers and garlic to wok and stir-fry for 1 minute. Add remaining 2 teaspoons peanut oil, shrimp mixture and onions and stir- fry for 1 minute. Spoon sauce and shrimp over watercress.