Fillet the tuna, put in a vacuum bag and bring it down to -18 degrees temperature to avoid high bacterial loads. Defrost tuna fillets, cut into tartare and place them on a baking sheet.
Meanwhile, in an old pot set fire to wood chips. Place the pan with the tuna in the upper part of the oven, just below put another pan full of ice. Turn off the wood chips covering the flame with the lid of the pot and immediately place in the lower part of the oven, immediately close the door and let the smoke saturated the cavity. Allow the fish to smoke for about 15 min.
For the granita mix the ingredients, put the mixture in the freezer and let it freeze. With a spoon, scrape to get a granita. Place the tuna on the plate cut with a pastry cutter and sprinkle with some salt flake. Garnish with some granita and mint leaves.