Place the diced tuna into a bowl with a generous splash of olive oil, some salt and pepper, the lime juice and zest, the onion, the ginger, and the coriander. Mix gently and add the mango. Mix gently but thoroughly and chill until needed (up to one day before serving).

To make the guacamole, scoop the avocado flesh into a bowl. Mash well until smooth, then season with olive oil, salt, and pepper. Stir in the lime juice and zest, onion, and coriander. Mix well until evenly combined.

To serve: Place a ring mold on each plate and half-fill with guacamole. Top with the tartare. Remove the molds and garnish with the reserved coriander leaves. (If you don’t have a ring, you can just do a scoop of avocado, flatten it out, and top with the tuna mixture.)