Cut lemon in half and squeeze out the juice. Reserve 1 tbsp of the juice and place the remaining juice in a small pot with 1 tbsp salt and the squeezed lemon halves, then add just enough water to cover. Bring to the boil, simmer for about 20 minutes, then leave to cool.
Select a large saucepan with deep sides. Add oil for frying, about 2-3cm deep. Heat oil to 180°C over a medium heat.
Using scissors, cut each rice paper round into 8 wedges. Carefully place a few in the hot oil and fry until crisp – they should only take a few seconds. Remove and drain on paper towels. Repeat for remaining rice paper wedges, making sure the oil is reheated after each batch.
With a paring knife, cut away the flesh and pith from the salted lemons and discard. Finely julienne the lemon skin.
Cut cucumber lengthways and, using a teaspoon, scoop out the seeds. Cut cucumber into small dice. Very finely slice the spring onion into rounds.
In a small bowl, combine the sea salt, chilli flakes, ground ginger and toasted black and white sesame seeds.
Dice tuna into 1cm pieces and place in a medium-sized bowl. Dress with extra virgin olive oil and reserved lemon juice. Add spring onion and cucumber and combine.
Place the mixture on to four plates. Garnish with the spice mix, coriander leaves and a small amount of the salted lemon. Serve the tuna with a bowl of the rice crisps to share at the table.