In a shallow dish combine the oregano, white and black sesame seeds, brown sugar, pinch of salt and pepper, julienned ginger, coriander (cilantro), Vietnamese mint, Asian basil and the pounded ginger, garlic, and onion. Mix well, and then roll the tuna steak in the mixture, turning to coat it well. Place a nonstick pan over medium-high heat, and add the vegetable oil. Sear the tuna for 45 to 60 seconds on all sides, or until browned. Remove the tuna and slice into 3/4-inch thick slices and serve on a bed of the mango salsa. Finish with a squeeze of lime and sprinkle with a pinch or black sesame seeds.