Trim or break off asparagus spears at tender point; rinse and cut into 2-inch diagonal pieces. Cook asparagus until crisp-tender, about 3 to 5 minutes. Do not overcook. Drain and rinse under cold water. Combine chicken broth, cornstarch, and soy sauce and set aside. Stir-fry halved scallops and mushrooms in oil with garlic until scallops are just cooked through, about 4 minutes. Stir in cornstarch mixture. Cook, stirring, until sauce thickens. Add drained asparagus, tomatoes, and green onions; heat. Pepper to taste. Serve over rice. Serves 4.