- 1 1/4 pounds very fresh (sushi-grade) fish fillets (such as tuna, hamachi, barramundi, or mahi-mahi), cut into 1/2-inch cubes
- Fine sea salt
- Freshly ground black pepper
- 3 tablespoons fresh lime juice
- 2 tablespoons fresh lemon juice
- 2 teaspoons sugar
- 1 cup chopped seeded tomatoes
- 3/4 cup chopped red onion
- 2 tablespoons sliced pickled jalapeño chiles from jar plus 1 tablespoon liquid from jar
- 1 tablespoon olive oil
- 1/2 large head of romaine lettuce, very thinly sliced crosswise (3 to 4 cups)
- 1 large avocado, peeled, pitted, diced
- 3 tablespoons chopped fresh cilantro
- 12 purchased crisp taco shells
- Purchased salsa
- Place fish in medium glass dish or bowl; sprinkle with sea salt and freshly ground black pepper. Add lime juice, lemon juice, and sugar and toss. Cover and chill until fish turns white and no longer looks raw, tossing occasionally, at least 4 hours and up to 6 hours.
- Strain fish, discarding marinade. Place ceviche in large bowl; add tomatoes, red onion, pickled jalapeños, 1 tablespoon liquid from jar, and olive oil and toss to blend. DO AHEAD: Can be made 2 hours ahead. Cover and chill.
- Add lettuce, avocado, and cilantro to ceviche mixture and toss. Fill taco shells with ceviche mixture. Top each taco with large spoonful of salsa and serve.