Step 1

Line a baking sheet with plastic wrap. Arrange the tuna slices in a single layer on the baking sheet and freeze until fairly firm, about 15 minutes.

Step 2

Stack the slices of tuna. Using a very sharp chef’s knife, cut the tuna into neat 1/4-inch cubes. Transfer the cubed tuna to a medium glass or ceramic bowl and stir in the red onion, lime juice and black pepper. Cover the tuna with plastic wrap and refrigerate for 1 hour, stirring gently with a plastic spatula every 15 to 20 minutes (the diced tuna will change color slightly).

Step 3

Just before serving, gently fold in the diced avocado and chopped cilantro. Season the ceviche with salt. Transfer the ceviche to a serving bowl or individual glasses. Garnish with cilantro leaves.