Begin by washing and soaking the sushi rice. Place the rice in a dish and rinse thoroughly with cold water, carefully washing and draining the rice several times until the water begins to run clear. Cover the rice again with water, then leave to soak for 20 minutes

Once the rice has soaked, carefully drain away the soaking water and place the rice in a pan with a tightly fitting lid. Pour over 180ml of water and place over a high heat, allowing the water to come up to to the boil

Reduce the heat to very low and cover the pan with its lid, leaving the rice to cook for 20 minutes

Meanwhile, prepare the sushi vinegar. Once the rice is nearly cooked through place the vinegar, sugar and salt in a large mixing bowl. Transfer the bowl to a microwave and heat for 10 seconds to warm through, then stir well until the sugar and salt have dissolved

Remove the cooked rice from the heat and add to the bowl with the sushi vinegar. Stir the rice carefully, until the rice evenly absorbs the sushi vinegar. Once mixed through, set the rice to one side and leave to cool to room temperature

While the rice is cooling, prepare the tuna for the nigiri. Using a very sharp knife, cut the tuna into 10 even rectangular pieces, each weighing 12g

Weigh out 10 12g balls of sushi rice and shape into oblongs slightly smaller than the tuna pieces. Cover each piece of rice with a slice of tuna and arrange the nigiri on a baking tray or board

Using a blowtorch, sear the nigiri evenly until lightly golden and just beginning to char. Arrange across a serving platter (or individual plates if preferred) and top each piece of nigiri with 2g of caviar, serving immediately with wasabi paste and sushi vinegar on the side