Preheat oven to 400ºF. Coat a baking pan with cooking spray.
Season fish with salt and black pepper, and put fish in pan.
Combine spinach, sour cream, onion and garlic powder; and stire until evenly mixed (A food processor would give a smoother and more finished looking result)
Spread spinach mixture evenly over cod in pan. Cover pan with foil and bake 10 minutes. Uncover and bake until fish is fork-tender, about 5 minutes more. Yields 1 piece of cod, about 1/3 cup of spinach mixture.
A side dish of sliced and steamed summer squash would be a nice accompaniment.