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.
Ready in 20 min.