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1
Soak and Flake the Cod
24 Hours Soaking + 10 Mins
Cold water, ~20°C (68°F)
Soak 500 g of salt cod in cold water for 24 hours, changing the water 3–4 times. Drain, pat dry, pull off the skin, pick out any bones, and flake the flesh into bite-size pieces.
Keep the flakes chunky — they should stay visible in the finished gratin, not dissolve into the onion.
Done when: Flakes are tender, lightly salted, and free of skin and bones.
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2
Poach the Cod and Soften the Onion
10 Mins
Gentle simmer, ~85°C (185°F); then medium heat, ~160°C (320°F)
Simmer the flaked cod in a pan of water for 4–5 minutes until it turns opaque, then drain well. Heat the olive oil in a wide frying pan over medium heat, soften the sliced onion for 4 minutes until translucent, add the garlic for 30 seconds, then stir in the drained cod and cook for 2 minutes until fragrant. Season with pepper and spread the mixture over the base of a buttered baking dish.
Do not brown the garlic — it turns bitter in seconds, so add it after the onion has softened.
Done when: Cod is opaque and flakes easily; onion is soft and sweet-smelling.
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3
Boil and Mash the Potatoes
20 Mins
Boiling water, 100°C (212°F)
Boil the potato chunks in salted water for 15–18 minutes until they break apart easily. Drain well and steam-dry for 1 minute, then mash with the butter and warm milk until smooth. Beat in the egg yolk, the nutmeg and the pepper, and taste — it should need little or no salt.
Mash while the potatoes are still hot and the milk is warm; cold milk or a cold bowl makes the mash gluey.
Done when: Mash is smooth, fluffy and holds a peak on the spoon.
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4
Assemble the Gratin
10 Mins
Room temperature, ~20°C (68°F)
Spoon the mashed potato over the cod and onion in the baking dish and spread it evenly right to the edges, sealing the fish underneath. Use the back of a spoon to make gentle swirls on the surface, then spread the mayonnaise over the top in an even layer.
Seal the mash to the edges of the dish — any gap lets the cod dry out in the oven.
Done when: The cod layer is fully covered; the mayonnaise coats the whole surface.
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5
Bake Until Golden
20 Mins
Oven at 200°C (400°F)
Bake at 200°C (400°F) for 18–20 minutes until the mayonnaise topping is golden brown in patches and bubbling at the edges, and the fish in the centre has reached 63°C (145°F).
If the top colours too fast, move the dish to a lower shelf — do not cover it, or the mayonnaise will steam instead of crisping.
Done when: Top is golden brown in patches and crisp at the swirls; the edges bubble.
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6
Rest, Garnish and Serve
5 Mins
Resting, ~20°C (68°F)
Rest the gratin for 5 minutes so the layers settle and spoonfuls hold together. Arrange the black olives over the top, scatter over the chopped parsley, and serve straight from the dish — about 425 g per person.
A wide spoon serves it best; a knife cuts through the soft layers.
Done when: Spoonfuls hold their shape on the plate; the topping stays crisp.
auto_awesome Finishing Touches & Plating
Let the gratin rest for five minutes before serving — the layers settle as it cools slightly, and the first spoonful holds together instead of sliding apart.
Portugal
50m
