Preheat the Oven : Preheat your oven to 375°F (190°C). Lightly grease a medium-sized baking dish with olive oil or butter.

Prepare the Spinach : In a large skillet, heat olive oil over medium heat. Add the fresh spinach and sauté until wilted, about 2–3 minutes. Transfer the spinach to the prepared baking dish, spreading it evenly to form a bed.

Arrange the Cod : Pat the cod fillets dry with paper towels and season both sides with garlic powder, onion powder, paprika, salt, and pepper. Lay the seasoned fillets over the spinach in the baking dish.

Make the Creamy Sauce : In a small saucepan over medium heat, melt the butter. Add the heavy cream and stir until warmed through. Gradually mix in the cheddar and Parmesan cheeses, stirring continuously until melted and smooth.

Assemble and Bake : Pour the creamy cheese sauce evenly over the cod and spinach. Bake in the preheated oven for 25-30 minutes, or until the fish turns opaque and flakes easily when tested with a fork.

Serve : Remove the casserole from the oven and let it rest for 5 minutes before serving. Serve warm, garnished with additional grated Parmesan or fresh herbs, if desired.