Peel, devein, and butterfly shrimp. Combine shrimp, 1 1/2 tablespoons sherry, and gingerroot. Cover and marinate in refrigerator 30 to 45 minutes.
Combine remaining 1 1/2 tablespoons sherry, cornstarch, chicken broth, soy sauce, and vinegar; stir well. Add ketchup and next 3 ingredients. Set aside.
Coat a wok or large nonstick skillet with cooking spray. Heat at medium-high (375°) until hot. Add walnuts, and stir-fry 30 seconds. Remove walnuts from wok, and set aside.
Drizzle peanut oil around top of wok, coating sides. Add spinach to wok; stir-fry 2 minutes. Remove to a serving platter; keep warm.
Add garlic to wok; stir-fry 10 seconds. Add sweet red pepper and 2 tablespoons water; stir-fry 2 minutes. Add shrimp mixture and green onions; stir-fry 3 to 4 minutes. Add cornstarch mixture; stir-fry until mixture is slightly thickened and bubbly. Spoon over spinach, and sprinkle with walnuts. Serve immediately.