1 Soak salt cod in water: Salt cod typically comes either in dry fillets, already boned and skinned, or it comes whole. The fillets need to be rinsed, then soaked in water, and kept chilled, for 24 hours, with one or two changes of the water.
If you are using a whole fish, not prepared fillets, it needs to be soaked in water for up to 48 hours, also with several changes of water, and the bones and the skin removed and discarded after soaking.
2 Simmer salt cod in milk and/or water: Put salt cod in a saucepan. Add enough milk, water, or a mixture of milk and water to just cover. Bring mixture to a simmer. Let simmer for a couple minutes. Remove the fish and set aside.
3 Parboil potatoes and slice: Parboil the potatoes for 20 minutes (you can cook them in the water you used to cook the fish if you want). Slice potatoes into 1/4-inch thick rounds.
4 Layer casserole with onions, potatoes, salt cod, olive oil, pepper, then top with hard boiled eggs and olives: In a large pyrex casserole or Dutch oven (use Dutch oven if making on stove-top), generously coat the bottom of the pan with olive oil. Place a layer of onion rings over the bottom of the pan.
Place a layer of sliced potatoes over the onions.
Breaking up the salt cod with your fingers, place pieces of salt cod in a layer over the potatoes. Taste the fish for saltiness. After a day of soaking and further cooking, there should be just a hint of saltiness in the fish. If most of the salt was soaked out of the fish, and the fish doesn’t taste at all salty, you may need to sprinkle some salt back on to the fish as you place the layers down.
Generously pour some olive oil over the fish. Sprinkle with freshly grated black pepper.
Repeat with another layer of onions, potatoes, fish, olive oil, pepper (and more salt if needed).
Then finish with layers of onions, potatoes, more olive oil, sliced hard boiled eggs, and olives.
5 Cook on stovetop or bake: Place on stove top on medium heat, and cook, covered, for 30 minutes. Or heat an oven to 350°F and cook, covered, for 30-40 minutes, or until everything is completely through.
Serve with sides of rice and salad.