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

Instructions

1. If using fresh yuca, remove peel and pink flesh using a vegetable peeler. Cover fresh or frozen yuca with salted cold water in a 3- to 4-quart saucepan. Bring to a boil, then simmer, covered, until yuca begins to split, 15 to 20 minutes. Transfer yuca with a slotted spoon to a cutting board, reserving water in pan. Cut into 1-inch wedges, pulling out and discarding tough core. Return yuca to cooking water and simmer, covered, until tender and completely translucent, 15 to 25 minutes more.
2. Make pickled onions while yuca cooks: Cover onion with boiling-hot water in a heatproof bowl and let stand 15 minutes. Drain onion and return to bowl. Add lime juice and salt, tossing to coat, and let stand at least 5 minutes before serving.
3. Cover onion with boiling-hot water in a heatproof bowl and let stand 15 minutes. Drain onion and return to bowl. Add lime juice and salt, tossing to coat, and let stand at least 5 minutes before serving.
4. Make fish stew while yuca finishes cooking: Bring water, tomatoes, cilantro, cumin, and 3/4 teaspoon salt to a boil in a 3- to 4-quart heavy saucepan. Reduce heat and simmer, covered, 5 minutes. Toss fish with remaining 3/4 teaspoon salt and stir into broth. Cook, uncovered, over moderate heat, stirring occasionally, until fish is just cooked through, 3 to 5 minutes. 3Drain yuca and divide among 4 large soup bowls. Ladle fish stew over yuca and top with some of pickled onions.
5. Bring water, tomatoes, cilantro, cumin, and 3/4 teaspoon salt to a boil in a 3- to 4-quart heavy saucepan. Reduce heat and simmer, covered, 5 minutes. Toss fish with remaining 3/4 teaspoon salt and stir into broth. Cook, uncovered, over moderate heat, stirring occasionally, until fish is just cooked through, 3 to 5 minutes. 3Drain yuca and divide among 4 large soup bowls. Ladle fish stew over yuca and top with some of pickled onions.