Whisk the 1/2 cup water with cornstarch. Add the oyster sauce and soy sauce. Whisk well.
Cook green beans with 2 tablespoons of water in covered bowl for 4 minutes in microwave.
In large skillet, heat 2 tablespoons of olive oil. Cook chopped onion for 3 minutes on lower heat stirring occasionally. Add garlic and cook another minute.
Add cod to skillet, cover, and steam for 3 minutes on each side. Remove only the cod from skillet and keep warm between 2 plates.
Add cornstarch mixture to skillet and heat. All green beans and simmer on lower heat for 3 minutes or until sauce thickens.
Serve the green beans in center of plates. Place cod on top of the green beans. Baste each portion of cod with remaining sauce in skillet.