Begin by desalting the cod overnight in cold water, changing the water several times for best results.
The next day, rinse thoroughly, then poach in abundant simmering water for 10 minutes. Drain, cool, and flake into strips, carefully removing all bones and skin.
Prepare the spinach: trim tough ribs and wilted parts, wash meticulously in several changes of water, then blanch in boiling salted water for 5 minutes. Drain and shock in ice water to retain its vibrant green color and crisp texture.
For the Mornay sauce—a velvety classic—melt half the butter over low heat in a saucepan. Stir in the flour to form a smooth roux, then gradually whisk in the milk. Simmer gently to thicken without boiling. Season with salt and pepper, stir in the grated Emmental until melted.
Off the heat, vigorously whisk in the egg yolks. Return to very low heat, stirring constantly to temper without scrambling. Finish off heat with the heavy cream for luxurious richness.
Preheat oven to 240°C (475°F, th. 8).
In a buttered gratin dish, layer the blanched spinach, top with cod strips, pour over the Mornay sauce, and sprinkle with remaining grated cheese and dots of butter.
Bake for about 15 minutes until golden and bubbling. Serve hot for an elegant, comforting dish drawing from French culinary tradition.
