Toss together the mango, onion, 1 tablespoon of the oil, lime juice, chilli and coriander and set aside.
Place the tuna in a food processor with the shallot and soy sauce. Briefly pulse until the mixture just comes together but isn’t puréed. Lightly wet your hands, then shape into 4 burgers.
Heat a griddle pan until smoking hot. Season the burgers and brush all over with the remaining oil. Cook the burgers on the pan for 1–2 minutes on each side, or until browned but still rare inside. Toast the ciabatta halves, then place a spoonful of mango salsa inside. Top with the burgers and serve.