Get a baking sheet and line it with some clingfilm. Group the tuna slices into one layer onto the baking sheet. Freeze the tuna until the slices are firm. Freeze for about 15 to 20 minutes.
Stack slices of tuna on top of each other. Using a chef's knife, slice the tune into cubes. Move the cubes of tuna to a medium sized glass. Stir the tuna cubes in lime juice, black pepper and onion. Cover the mix of tuna with clingfilm and put in the fridge an 1 hour. Stir this gently with a spatula every 15 minutes. You will know when it's nearly done by the slight change of colour in the tuna.
Before you serve, put in the chopped avocado and cilantro. Season with some salt. Move the ceviche to a bowl. Decorate with cilantro leaves