Wash and dry the oranges and lemons well. Peel 1 orange and 1 lemon, making sure you avoid the white pith of the peel which is bitter. Then chop the orange and lemon peels into very fine Julienne slices.
Squeeze the other orange and 1 lemon and pour the juice in a pan with the Julienne slices, the olive oil, salt and pepper.
Heat up the citrus sauce over low heat, mixing constantly until it becomes slightly thick.
Cut the tuna into very thin slices like beef carpaccio. Slice the remaining lemons and make a bed of lemon slices on a serving dish. Place the tuna slices on top. Pour the citrus sauce over the tuna, covering the fish uniformly. Garnish with the capers and mint leaves and let it sit for 10 minutes before serving.