Preheat oven to 400 degrees F and lightly grease a 9x13 baking dish.
Lay cod fish filets out in the prepared baking dish. Season fish with salt and pepper.
In a small bowl, stir together the butter, parmesan, flour, garlic, basil, onion powder, dijon mustard, and lemon juice.
Top each fillet with a large spoonful of the butter mixture.
Bake in the preheated oven for about 10 minutes. Actual cooking time will depend on the thickness of your fish. The fish is done when it flakes easily with a fork.