For the tuna, mix the lemongrass, soy sauce, lime juice and vegetable oil in a bowl until well combined. Add the tuna steaks and marinate in the fridge for one hour.
Heat a griddle pan until very hot.
Sear each piece of tuna for 30 seconds on each side, then remove from the pan.
Meanwhile, for noodle salad, pour boiling water over the noodles and leave to soak for five minutes.
Place the lime juice, garlic, ginger, fish sauce, chillies and peanuts into a pestle and mortar and crush to a fine paste.
Drain the noodles and toss with two-thirds of the dressing. Set aside for 5-10 minutes to allow the flavours to infuse. Add the spring onions, holy basil and mint and mix until well combined.
To serve, carve the tuna into thick slices. Pile the noodle salad onto serving plates and top with the tuna slices.