Cut the tuna very finely into cubes (with a sharp kitchen knife, not a cutter).
For the marinade, mix olive oil, soy sauce, lime juice, a tiny bit of ginger, cilantro, wasabi paste and a tiny bit of brown sugar and mix with the tuna. Let the tartar sit for about 10 minutes, then bring to the table.
Roast the tuna in one piece so that it is crispy on the outside but still raw on the inside. Cut into slices and drizzle with the marinade. Serve with a raw mango chutney.
Mango chutney: Finely dice the mango and mix with the remaining ingredients. Serve with the tuna.