Wash the tuna and dab dry. Divide the lemongrass blade and put two pieces through each steak. Mix tandoori spice, yoghurt and sugar. Spread the steaks all around and leave them covered and marinated in a cool place for about 2 hours.
Place the tuna on a greased baking tray and cook in a preheated oven (electric: 200 °C/ gas: level 3) for 20-25 minutes. In the meantime, put the rice in plenty of boiling salted water, cover and let it swell at low heat for about 20 minutes.
Wash the tomato, quarter it, remove the seeds and cut into small cubes. Arrange fish and rice on plates. Sprinkle rice with tomato cubes. Sprinkle fish with lemongrass strips and grated lime zest to taste and serve garnished with lime wedges and lemon balm.