The tuna is quickly marked on both sides on the grill so that the interior stays raw, giving it a two-color effect. Cooking it on the barbecue is instantaneous. If you don't want to use a barbecue, it is best to sear the tuna in a very hot pan. Oven cooking would take too long and would destroy the textures.

1. Chop some of the tuna for the tartar; marinate it for 2 to 3 hours in the citrus marinade;
2. take a slice of tuna and flatten it between two sheets of waxed paper; brush with olive oil;
3. line each metal ring with a slice of tuna - the ring should be about as high as your little finger;
4. place a spoonful of the tuna tartar in the middle; fold the slice over to enclose;
5. mark the tuna on both sides on a very hot grill, about 15 to 20 seconds per side; remove; the inside will still be cold and raw. Since this takes only a matter of seconds, sear the tuna only at the last minute in order to serve it warm;
6. pour a little of the citrus and lemon balm sauce onto plates; place the rare tuna on top; remove the metal ring; garnish with a few lemon balm leaves and serve immediately.