Squeeze the grapefruit juice and pour into a sandwich bag with the fish sauce. Add the piece of tuna. Tie up the bag, squeezing out most of the air so the tuna is completely covered in the juice. Leave for 40 minutes, after which time the outside of the tuna will be pale and "cooked". Now carefully pour the grapefruit juice from the bag into a bowl, dry off the tuna and put to one side.
For the dressing, mix the sesame oil, olive oil and chillies into the grapefruit juice. Use as much chilli as you like, and season to taste. Tear off a good handful of coriander and mint from the bunches and put to one side to use for garnish later. Finely chop the remaining herbs and really pat these around the tuna to encrust it. Wrap in cling film and place in the fridge until needed.
Boil the noodles for 1 minute until they are slightly flexible, drain and allow to steam dry and cool. Add a little olive oil to a hot non-stick pan, add your noodles and leave them until they are nice and crisp on one side. Now flip them over and do the same on the other side – it doesn't matter if some stick to the pan, just scrape them up and turn them over. Divide the crispy noodles between 4 plates. Slice your tuna up about 0.5cm thick – in Japan it's a sign of generosity to have nice thick slices of tuna, but I like them a little thinner as they are more delicate in the mouth.
Place the tuna on the noodles, sprinkle over the torn-up herbs that you put to one side earlier, sprinkle with the spring onions and then drizzle a couple of spoonfuls of the dressing over the tuna. Before your eyes you will see the cut sides of the slices of fish begin to change colour and "cook". Serve straight away.