Preheat oven to 325°. Coat a shallow baking dish with nonstick cooking spray.
Arrange fish in prepared pan; sprinkle with salt and pepper to taste. Scatter onion over fish.
In a small bowl, combine lemon juice and butter; pour over fish and onions. Sprinkle with paprika. Bake, uncovered, for 30 minutes. Garnish with parsley if desired and serve.
Yield: Four servings, each with 2 grams carbohydrates, trace of fiber, 25 grams protein.