1. Preheat oven to 375°F. Cut cod into 4 portions. Coat a shallow baking dish with non stick cooking spray. Scatter onion over bottom of dish.
2. Stir sour cream and remaining ingredients (except cod fillets) together in a bowl until smooth. Coat fillets with sour cream mixture. Put fillets on top of the onions and spread remaining sauce evenly over top of cod.
3. Bake fish 20 to 30 minutes, depending on thickness, until center turns opaque. Garnish with thin slices of lemon and parsley.