Thaw fish, if frozen. Rinse fish; pat dry. Measure thickness of fillets; set aside.
In a large skillet cook onion in hot oil until tender but not brown. Add tomatoes, corn, vinegar, and sugar. Bring to boiling. Carefully add fish fillets. Return to boiling; reduce heat. Simmer, covered, for 4 to 6 minutes per 1/2-inch thickness of fish or until fish just flakes with a fork.
To serve, place couscous on a platter; arrange fish mixture atop. In a small bowl stir together the walnuts, bread crumbs, parsley, and melted margarine or butter; sprinkle over fish. Makes 4 servings.