Preheat the oven: Preheat your oven to 375°F (190°C).

Cook the spinach: In a large pan, melt the butter over medium heat. Add the chopped onion and garlic and sauté for 5 minutes until softened. Stir in the spinach and cook until wilted, about 3 minutes. Season with salt and pepper.

Prepare the haddock: In a baking dish, place the smoked haddock fillets. Season with salt and pepper.

Make the creamy sauce: In a saucepan, heat the heavy cream over medium heat. Stir in the Dijon mustard (if using) and 3/4 cup of the shredded cheese. Stir until the cheese is melted and the sauce is smooth.

Assemble the bake: Layer the sautéed spinach over the haddock fillets in the baking dish. Pour the creamy cheese sauce evenly over the top, then sprinkle with the remaining 1/4 cup of shredded cheese.

Bake: Bake in the preheated oven for 20-25 minutes, until the top is golden brown and the fish is cooked through.

Serve: Garnish with fresh parsley and serve hot.