Preheat oven to 400℉ (200℃) and lightly grease a 9x13 baking dish.
Lay 4 (6 ounce portions) cod out in the prepared baking dish. Season fish with 1/2 teaspoon salt and 1/4 teaspoon black pepper.
In a small bowl, use a fork to mash together 1/4 cup butter (softened), 2 tablespoons freshly grated Parmesan cheese, 1 tablespoon all-purpose flour, 1 tablespoon minced garlic, 1 teaspoon dried basil, 1/2 teaspoon onion powder, 1 teaspoon dijon mustard, and 1/4 cup lemon juice.
Top each cod fillet with an equal amount of the butter mixture, spreading it evenly across the top.
Bake in the preheated oven for 10–12 minutes, or until the fish flakes easily with a fork and is opaque throughout.
Serve immediately, spooning any extra sauce from the baking dish over the fillets. Best served hot with optional lemon wedges or fresh herbs for garnish.