Put the salt cod in a deep dish or a bowl, pour in enough milk to completely immerse the fish, cover tightly with plastic wrap, and soak in the refrigerator for 24 hours. Drain and rinse fish, then flake it with a fork or your fingers.
To soak the beans, combine the lima, kidney and cannellini beans in a large pot or bowl, cover with cold water by 2 to 3 inches and let it soak overnight at room temperature. Drain when you’re ready to make the stew.
To make the stew, melt the butter in a stockpot over medium heat and add the onion. Cook until translucent, about 10 minutes. Add garlic and cook for 2 to 3 minutes more.
Stir in achiote paste, cumin and oregano and cook, stirring frequently, until very fragrant, about 5 minutes. Add the stock, milk, drained soaked beans and simmer, uncovered, until beans are just tender, 1 1/2 to 2 hours.
Stir in lentils, rice, zucchini, butternut squash and salt cod and simmer for 20 minutes more.
Season to taste with salt and pepper. Add black cod, fava beans, peas and corn and cook just until black cod is cooked through (firm and opaque), 3 to 5 minutes.
To serve, ladle the fanesca into eight bowls and garnish each with a little cabbage, parsley, and cilantro, a few jalapeños and fried plantains, a wedge or two of hardboiled egg, and a few slices of avocado.
Drizzle each with olive oil. Alternatively, offer all of the garnishes in little bowls to be added to taste at the table, along with the ají.