Arrange the fish fillets in a large pot. Add the chopped soup vegetables. Season with salt and pepper and cook over high heat.
Meanwhile, prepare the vegetables.
Shred the carrot coarsely (use the side of your grater with the large “raindrop” holes). Thinly slice the onion and finely chop the garlic.
Sauté the onion and bay leaf in a pan, in a small amount of olive oil. Add the carrot and garlic and stir for a minute. Remove from heat and season with salt.
Peel and coarsely shred the potatoes. Squeeze the excess liquid using the palms of your hands. Fry in the remaining olive oil, stirring constantly lest it should burn. When a crust starts to form, remove from heat and combine with the carrot and onion (you can discard the bay leaf).
Take the fish out of the pot. (You can strain the liquid and cook a handful of rice in it thus turning it into soup that will make a delicious starter!) Use two forks to carefully shred the fish and remove any bones.
Add the fish to the vegetables.
Preheat the oven to 160 °C.
Meanwhile, prepare the sauce.
Melt the butter in a small pot. Stir in the flour. Add the milk and then the cream, stirring constantly.
Bring to a boil stirring constantly (lest lumps should form). Remove from heat and season with nutmeg and pepper.
Place the egg yolks in a small bowl. Stir in a small amount of the sauce, adding one teaspoon at a time. By doing so, the egg yolks will adjust to the temperature and will not curdle. Add the egg yolks into the sauce. Stir well and season with lemon juice and salt to taste.
Combine one half of the amount of the sauce with the fish and vegetables.
Spoon into an oven-proof dish and pour the remaining sauce on top.
Sprinkle with crushed corn flakes or breadcrumbs.
Bake for 25-30 minutes.