1. Preheat the oven to 190°C (375°F).

2. Line a baking dish with parchment paper for easy cleanup.

3. Season the cod fillet or loin generously with salt and freshly ground black pepper on both sides.

4. Place the seasoned cod in the prepared baking dish.

5. Bake the cod for 12 to 15 minutes, or until it is cooked through and flakes easily with a fork. Cooking time may vary depending on the thickness of the fish.

6. While the cod is baking, make the creamy egg and parsley sauce.

7. In a medium saucepan, melt the unsalted butter over medium heat.

8. Add the all-purpose flour and whisk constantly until smooth and bubbly, forming a roux. Cook for 1 to 2 minutes, stirring frequently to prevent burning.

9. Slowly pour in the whole milk, whisking constantly to break up any lumps and create a smooth sauce. Continue whisking until the mixture comes to a boil.

10. Reduce the heat to low and let the sauce simmer gently for 5 minutes, stirring occasionally, until thickened.

11. Remove the saucepan from the heat and stir in the heavy cream, fish stock (or water), freshly squeezed lemon juice, and a pinch of salt and pepper. Mix well to combine.

12. Add the chopped hard-boiled eggs and fresh flat-leaf parsley to the sauce, stirring gently to distribute evenly.

13. Taste the sauce and adjust the seasoning with more salt and pepper if needed.

14. When the cod is done, carefully transfer it to individual serving plates using a spatula.

15. Spoon the creamy egg and parsley sauce generously over the top of each cod portion.

16. Serve the baked cod with egg and parsley sauce immediately, garnished with additional fresh parsley if desired. Enjoy your comforting and flavorful meal!