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
Line a small baking sheet with parchment. Slice the tuna in ½-inch slices, place on the sheet and freeze for 15 minutes. Cut the tuna in ½ dice and chill.
Meanwhile, place onion in a medium bowl; cover with cold water, let stand 15 minutes and then drain.
Dice the avocado and add to the bowl with the tomato, serrano, 2 tablespoons lime juice, and 2 generous pinches salt; toss well. Add the tuna, drizzle the remaining 3 tablespoons lime juice and the oil on top, toss gently then refrigerate for 15 minutes.
Add the cilantro to the tuna mixture, season to taste and toss gently. Serve the ceviche on leaf lettuce with tortilla chips.