Cataplana is the name for both an array of Portuguese seafood dishes and for the clam shell–shaped vessel in which they are cooked. Here, a saucepan with a tight-fitting lid stands in for the more traditional pan. If you cannot find linguiça, a mildly spicy smoked Portuguese pork sausage, any high-quality fresh spicy Italian sausage can be substituted. Make sure the aioli is at room temperature just before serving, as it will blend in better and not cause the soup to cool.