Score both sides of trout with 3 diagonal slashes 1/4 inch deep and 1 inch apart. Combine cornstarch, salt and pepper; coat trout with mixture, shaking off excess. Heat 2 tbsp. oil in large nonstick skillet over medium heat until hot. Add trout; cook 6 to 7 minutes. Turn trout over; pour remaining 1 tbsp. oil around edges of pan. Cook 6 to 7 minutes longer, or until fish flakes easily when tested with fork. Meanwhile, blend sweet and sour sauce, 6 tbsp. water and sugar in small saucepan; set aside. Remove trout to serving platter and keep warm. Heat sweet and sour sauce mixture over medium-high heat until hot, stirring occasionally. To serve, pour half of sauce over trout and sprinkle green onions over fish and sauce. Pass remaining sauce.