Coat the tuna steaks in the grated ginger and sesame oil. Cover and leave to marinade for around an hour in the fridge.
Meanwhile, make the noodle salad. Cook the noodles as to the packet instructions, adding the sugar snap peas to the boiling water for the last 2 minutes of cooking time. Drain, rinse under cold water until cool and set aside.
To make the dressing, whisk together the lime juice, vinegar, sugar, fish sauce, sesame oil, garlic and chilli flakes. Add the spring onions. Put the noodles and sugar snap peas in a large bowl and pour over the dressing. Add the chopped coriander and chilli and mix well to combine.
To cook the fish, spread the sesame seeds out on a large plate and toss the tuna steaks until they are coated in a crust of sesame seeds. Heat the oil in a griddle pan and fry the steaks for 1-2 minutes on each side for rare, or 3-4 minutes per side if you prefer them more well done.
Serve the tuna with the noodles and garnish with extra coriander.