Cut the tuna steaks into small square slices, and squeeze the limes over them to start curing the tuna.

Add the soy sauce and Maggi seasoning sauce (I used 1 tablespoon each, but adjust to taste). Finely chop the onion and add it. For the cucumber, remove the seeds, dice it, and add it in. Do the same with the mango.

Mix well and taste. If it's too sour from the lime, add more soy sauce and Maggi seasoning sauce in equal parts to balance the flavor.