How to Make It
Put the salt cod fillet in a large pot of cold water, cover and refrigerate for 2 days. Drain the fish and pat dry. Cut the fish into 1-inch pieces.
In a large, deep skillet, heat 1/4 cup of the olive oil. Add the onion and half of the garlic and cook over moderate heat until softened, about 5 minutes. Add the salt cod and cook until the onion is lightly browned, about 5 minutes. Add the wine and cook until nearly evaporated, about 5 minutes. Add the potato and 2 cups of water and simmer until the potato is tender, about 15 minutes. Using a slotted spoon, transfer the cod and vegetables to a blender along with 1 cup of the cooking liquid. Blend until very smooth.
Wipe out the skillet and heat the remaining 2 tablespoons of olive oil in it. Add the scallions and the remaining garlic and cook over moderate heat until softened, about 3 minutes. Add the prosciutto, peas, parsley and lemon zest. Stir in the cod puree, cover and keep warm.
In a large pot of boiling salted water, cook the penne until al dente. Drain the pasta, reserving 1 cup of the cooking water. Add the pasta and cooking water to the skillet and cook, stirring, until the pasta is coated with a thick creamy sauce, about 2 minutes. Transfer the pasta to bowls and serve right away.