For the Sushi Rice
380g sushi rice
600ml cold water
For the Sushi Vinegar
½ cup rice vinegar
¼ cup sugar
½ tsp salt
½ vegetable stock cube, crumbled
Dash of nam pla (fish sauce)
To Assemble the Onigirazu
4 nori sheets
For the Sweet Potato and Omelette Filling
1 medium/ large sweet potato, peeled
2 tbsp katsup mani
½ tsp sesame oil
1 tsp rice vinegar
1 tsp mirin
1 tsp vegetable oil
2 eggs, lightly beaten
½ red pepper, thinly sliced, lengthwise
Pickled ginger slices
Kewpie mayonnaise (Japanese mayonnaise)
For the Salmon and Avocado Filling
100g smoked salmon
A handful of baby spinach leaves
1 ripe avocado, peeled and thinly sliced
For the sushi rice, place the rice in a saucepan and cover with the measured water. Leave to soak for at least 30 minutes, but can be left for up to 5 hours. Bring the rice to the boil in the saucepan then transfer to a solid steam container and place in the steam oven. Steam at 100°C for 15 minutes.
Meanwhile, put the ingredients for the sushi vinegar into a small pan and heat to dissolve the sugar and salt. Strain through a sieve to remove any stock cube residue.
Place the steamed rice into a large, shallow bowl. Put a quantity of ice cubes into another bowl of similar size and sit the bowl of rice on the ice. Gradually pour in the sushi vinegar and mix with a large spatula, cutting through the lumps of rice. Do not stir or else the rice will become crushed and lumpy. Fan the rice with the other hand while continuing to cut through with the spatula until the rice cools to room temperature. The rice will be shiny and sticky but each grain will be separate. Cover with a damp cloth and set aside.
For the sweet potato filling, preheat the oven on Fan plus, 210°C. Cut through the middle of the sweet potato to make ½cm thick discs. In a small bowl, mix together the katsup mani, sesame oil, rice vinegar, mirin and vegetable oil to make a marinade. Generously brush both sides of the sweet potato discs with the marinade and place on a sheet of non-stick paper on the baking tray. Put the tray in the oven and bake the sweet potato for 20 minutes turning them over after 10 minutes. Set aside to cool and season lightly with a little salt.
Heat a little vegetable oil in a small omelette pan and pour in the beaten egg. Cook gently to make a small flat omelette. Remove from the heat and allow to cool.
To assemble the onigirazu, put a square of cling film slightly larger than the nori sheet on to the work surface and then place the nori sheet on top, shiny side down and rotated 45° in relation to the cling film. Wet your hands (keep a small bowl of water handy) and take a small handful of rice. Place it in the middle of the nori sheet and shape it into a 9cm square keeping the layer of rice as even and compacted as possible.
For the sweet potato filling, drizzle a little kewpie mayonnaise over the rice and top with a thin layer of pickled ginger, place on a sweet potato slice then cut a square out of the omelette and place on top of the sweet potato. Top with the slices of red pepper. Drizzle over some of the okonomiyaki sauce. Finish with a final layer of sushi rice.
Dampen the edges of the nori sheet and fold in all four corners to make a wrapping over the rice. The finished result will look like a small parcel. Finally, gather up the cling film fairly tightly around the sandwich. Put something reasonably heavy (like a small breadboard) on the onigirazu to compress it slightly. Leave for 15 minutes to let the seaweed soften. When ready to eat, remove the cling film and cut the onigirazu in half or thick slices.