Desalt the Salt Cod: place the cod in a large bowl, cover with cold water and refrigerate for 24–48 hours, changing the water every 6–8 hours until it tastes mild. If short on time, rinse well and simmer briefly before proceeding.
Prepare the potatoes: peel (if desired) and cut the Potatoes into 1/4–1/2-inch slices or thin cubes. Pat dry and fry in hot Vegetable Oil in a skillet until just tender and lightly golden; drain on paper towels and set aside.
Poach and flake the cod: after desalting, place the cod in a pot with cold water, add a Bay Leaf, bring to a gentle simmer and cook until the fish flakes easily, about 8–10 minutes. Drain, remove skin and bones, and flake the flesh into large pieces.
Cook the aromatics: in a large sauté pan heat Olive Oil and gently soften the sliced Onions until translucent and sweet, then add minced Garlic for the last minute. Toss the flaked cod into the onion mixture and warm through, seasoning lightly with Salt and freshly ground Black Pepper.
Make the béchamel: melt Butter in a saucepan over medium heat, whisk in All-Purpose Flour to form a smooth roux and cook 1–2 minutes without browning. Gradually whisk in warm Whole Milk and Heavy Cream until silky and slightly thickened; simmer briefly, then finish with a pinch of Ground Nutmeg and adjust seasoning.
Assemble the casserole: in a buttered baking dish layer half the fried potatoes, spread the cod-and-onion mixture over them, then top with the remaining potatoes. Pour the béchamel evenly over the top.
Top and bake: sprinkle grated or torn Mild Melting Cheese over the béchamel. Bake in a preheated oven at 200°C (400°F) for 20–30 minutes, until bubbling and golden on top. If you like a deeper crust, finish under the broiler for 2–3 minutes, watching carefully.
Rest and serve: let the dish rest 5–10 minutes for the sauce to settle, then serve warm, spooning generous portions so each plate gets creamy sauce, potatoes and flakes of cod.
