Wash and pat dry fish with paper towels. Cut two diagonal slits on each side of the fish. Lightly sprinkle some salt in stomach cavity and both sides of fish.
Heat 2 tablespoons vegetable oil in a deep pan. Lower fish into pan and allow it to fry for 12 to 15 minutes on medium heat. Turn fish over and allow it to fry for another 12 to 15 minutes or until both sides of fish are golden brown and cooked through.
Transfer fish to a plate. Sprinkle julienned cucumber over fish.
Mix all sauce ingredients in a small bowl.
Heat remaining tablespoon of oil in a small saucepan. Add onions and garlic and fry till fragrant, about 1 to 2 minutes. Pour in sauce and season with a little salt. Cook for 2 to 3 minutes or until sauce is thick and bubbly.
Pour sauce over fish and serve immediately.