Prepare the tuna – rinse under water and pat dry with a paper towels, cut into 1 cm or smaller cubes, put to a container and cool in a fridge, while you’re cooking the rest.
Cut the onion into small cubes and put in cold water for 5 minutes to remove excess spiciness. Drain the water and dry the onion on paper towels.
Squeeze lime juice, peel and dice an avocado and a garlic, rinse and dry capers. Cut the chilli in 2 halves, remove membranes and seeds and cut into small pieces. In a separate bowl, mix the onion, garlic, chili, capers, avocado, salt, pepper, and 2 tablespoons of lime juice. Add the tuna.
In a separate jar, mix olive oil and the rest of lime juice, pour over the tuna and mix all the ceviche. Cool for 15 minutes, but not more – the tuna turns gray from prolonged contact with juice. Sprinkle with chopped greens, for example, cilantro. If you want, serve on lettuce leaves and garnish with microgreens and sesame seeds.