Fish and Yuca Stew with Pickled Onions

Name: Fish and Yuca Stew with Pickled Onions
Ingredients: 1 1/2 lb fresh or frozen yuca (1 large), cut crosswise into 2-inch sections for pickled onions; 1 medium red onion, halved lengthwise, then thinly sliced; 1/4 cup fresh lime juice; 1/2 teaspoon salt; 3 cups water; 1 lb tomatoes, coarsely chopped; 1 tablespoon chopped fresh cilantro; 1 teaspoon ground cumin; 1 1/2 teaspoons salt; 2 lb skinless tuna, bluefish, or mackerel fillets, cut into 1-inch pieces; Accompaniment: lime wedges; Garnish: chopped fresh cilantro