Firstly, prepare yoghurt sauce by incorporating yoghurt, minced lemongrass leaves, grated black pepper, the juice of a half lime and a little oil in a bowl. Mix and keep away.
Peel and cut avocado in slices; then place the slices on a dish. Drizzle evenly with the remaining lime juice.
Drain tuna and separate fillets by following the muscle shape, according to its structure.
Cut sandwiches in halves and place avocado slices on a side. Cover with yoghurt sauce and complete with tuna fillets.
If you wish, you can add some extra lemongrass leaves before completing and serving the sandwich.