The traditional zuppa di pesce that you most likely encountered in Italian-American restaurants was based on garlic and tomato sauce, which was simmered along with assorted fish to make a savory dish. Sometimes the sauce was used to dress pasta and the shellfish and finfish would be enjoyed as a second course. This version is more in a brodetto style, lighter and clearer than the traditional version, with saffron as a flavoring ingredient. This kind of preparation can be found with slight variations along the entire coast of Italy from Liguria to the Adriatic coast to the heel, Puglia, and the island of Sicily. I have given you the recipe with fish fillet, although traditionally zuppa di pesce is made with slices of whole fish with bones and skin intact. But it is tricky to eat that way even though the flavor is more complex.
Meanwhile, add the mussels to the soup base remaining in the saucepan. Increase the heat to high, cover the saucepan, and steam over medium heat, shaking the pan occasionally, until the mussels open, about 3 minutes. Discard any mussels that do not open on their own.
Stir the shrimp, parsley and steamed mussels into the large pot of soup. Simmer until the shrimp is cooked through, about 1 minute. Check the seasoning, adding salt, if necessary, and freshly ground black pepper. Garnish with chopped parsley. Ladle soup into warmed soup bowls. Serve with crusty Italian bread.
Drain the beans and transfer them to a 2-quart saucepan. Pour in enough water to cover by two fingers and drop in two of the rosemary sprigs. Bring the water to a boil, and then lower the heat so the water is at a bare simmer. Cook until the beans are tender but not mushy, with just enough liquid to cover them, 30 to 40 minutes. (If necessary, add more water a tablespoon at a time to keep the beans covered as they simmer.)
Remove the beans from the heat and gently stir in the oil, salt to taste, and the remaining 2 rosemary sprigs. Let the beans stand to cool and absorb the cooking liquid. The end result should be tender beans with a creamy consistency in just enough liquid to coat them. As the beans cool, taste and season with salt, if necessary.