Preheat the oven to 400°F (200°C). Lightly grease a 13 x 9 x 2-inch baking dish.
Place the 3 cod fillets (cut in half) in a single layer in the prepared baking dish and season both sides with salt and pepper to taste.
In a small bowl, whisk together 1/4 cup melted unsalted butter, 1 tablespoon olive oil, 3 minced garlic cloves, and 1 tablespoon Cajun seasoning until combined.
Brush or spoon the butter mixture evenly over the top of the cod fillets.
Bake uncovered for about 15 minutes, or until the fish is opaque and flakes easily with a fork.
Remove from the oven and serve immediately; garnish with chopped parsley and lemon slices if desired.
Notes
Use fresh or thawed cod for best texture.
Adjust Cajun seasoning to control spice level.
Ensure fillets are in a single layer for even cooking.
Let melted butter cool slightly before mixing if very hot.