In a skillet, heat olive oil and butter until butter is melted. Fry fish over medium heat for 4 minutes on each side or until fish flakes easily with a fork. Remove to a plate and keep warm.
Add garlic and ginger to skillet; cook and stir for 1-2 minutes. Stir in the sugar, vinegar and soy sauce. Combine cornstarch and water until smooth. Stir into skillet. Bring to a boil; cook and stir for 1-2 minutes or until thickened. Pour over fish. Sprinkle with chives and onion if desired.