Desalt the cod overnight.
The next day, rinse it and poach it for 10 minutes in plenty of water. Drain it, let it cool and cut it into strips. Remove bones and skin.
Peel the spinach:remove the ribs and the withered parts, wash them in several waters and immerse them in boiling salted water for 5 minutes. Drain and refresh them immediately to keep them nice and green.
Prepare the sauce:melt half the butter over low heat, add the flour, whisking constantly. Gradually pour in the milk and let it reduce without ever letting it boil. Add salt and pepper. Add the grated cheese and melt it.
Remove the sauce from the heat, add the egg yolks, stirring well. Put back on very low heat while continuing to whip, then, off the heat, add the fresh cream.
Heat the oven to 240°C (th.8).
In a buttered gratin dish, arrange the spinach, then the stripped cod, drizzle with sauce, sprinkle with grated cheese and the rest of the butter in pieces.
Slide the dish into the oven for about 15 minutes.
