Make the best mashed potatoes you can. Boiled potatoes (20 minutes) mashed by hand (see photo).
Melt a good slab of buttering the potatoes, a little bit of the cooking water, crème fraîche (150g?), 150g of grated gruyère, salt (not too much, the cod will add its own salt, pepper and grated nutmeg. That's for the potatoes.
Fry two finely chopped onions in butter with a little oil so the butter doesn't burn for 20 minutes medium low heat in a pan. Add the frozen spinach (leaves, not shredded) the juice of one lemon, a little salt, pepper. When the spinach are warm, it's done. Now your spinach are ready.
Soak the salted cod (use the soft variety, there's more meat and less bones) in water for 24 hours (up to two day depending on the thickness of the cod) changing water every 8 hours. Cut in big pieces and steam for 20 minutes (or cook in milk). Shred the cooked cod and remove the bones. Your cod is happily ready to mix with its friends...
Spread one layer of mash in a gratin dish. Then the spinach. Add the cod, finish with a layer of mash. Cut your mozzarella in slices. Dispose the slices on the top.
Perfect with a green or a tomato salad.
