Place the chickpeas in a heavy-bottomed pot and just cover with cold water. Add the baking soda and cover. Leave the chickpeas to soak overnight. When they are ready to be used, transfer the chickpeas and the soaking water to a large saucepan. Salt, to taste, and cook over a medium flame until tender. Cut the onion in 1/2 through the root and slice them into half moons, each 1/8-inch thick. In a second large saucepan, heat the 1/4 cup of olive oil over medium heat. Add the onions and cook until soft and golden brown. Add the garlic, tomatoes, chili flakes, and salt and pepper, to taste. Add the chickpeas and water and leave to simmer until the flavors have melded, for at least 30 minutes and up to 60 minutes. Before serving, chop the anchovies and add the parsley. Toast the bread slices, rub surface with the remaining garlic clove and spread on the anchovy paste. Drizzle them with olive oil and serve with the soup.
Recipe courtesy of Mario Batali