The night before you plan to cook the stew, combine the dried chickpeas and baking soda in a bowl and cover with cold water. Allow to soak overnight, then drain and rinse the chickpeas.
In a large saucepan, combine the chickpeas with 2 1/2 quarts of water and bring to a boil. Reduce the heat to low and simmer until chickpeas are tender, about 2 hours. Every 10 minutes or so, add 1/4 cup of cold water to slow down the simmer. By the time the chickpeas have cooked, the water should have reduced so it barely covers the chickpeas. Remove from the heat and set saucepan aside.
Heat the olive oil in a small sauté pan over medium-low heat. Add the garlic and cook until browned on all sides, about 3 minutes. Transfer the garlic to a mortar. Add the bread to the pan and brown on both sides, about 1 minute on each side, then transfer the bread to the mortar with the garlic and set aside.
Remove the pan from the heat and allow the oil to cool for a few minutes. Add the pimentón, saffron, cumin and vinegar to the pan and mix well.
In the mortar, mash the toasted garlic and bread into a thick paste.
Return the chickpeas to the stove and bring to a boil over medium-high heat. Reduce the heat to low and stir in the pimentón mixture and the garlic-bread paste. The liquid should become a thick, stewy sauce. Simmer for 5 minutes.
Slice the cod into 1”-thick pieces and add them to the stew. Simmer for about 6 minutes or until cod is cooked through. Season to taste with salt and pepper and serve immediately.
Note: You may find salt cod already rehydrated and desalinated, allowing you to skip the first step. You can also use good-quality canned chickpeas for this recipe, if you like. Substitute 3 cups of the canned chickpeas for the dried chickpeas and skip the second and third steps of soaking and cooking the chickpeas.