Tuna Ceviche with Avocado and Serrano Chiles

Ingredients

• 1 pound sushi-grade tuna
• 1/2 small red onion, finely diced
• 1 ripe Hass avocado
• 1 small, ripe plum tomato, seeded and finely diced
• 1 serrano chile, seeded and minced
• 5 tablespoons fresh lime juice, from 2 to 3 limes
• 3 tablespoons extra-virgin olive oil
• Coarse sea salt and freshly ground pepper
• 3 tablespoons finely chopped cilantro